Alternate realities, dimensions, realms, or planes of existence aren't uniform. This goes beyond differences to the overall alignment, the residents, the size and geography, or even the “rules” of the dimension. On top of all these; every reality sits on a level on the cosmic chain of limits. Every realm or place that can exist exists at a state of highness and energy- some scholars believe the chain is limitless in both directions, while some think that there is a definite bottom- a lifeless, lightless pit of nothingness where the great evils will one day be banished forever.
However, all of these limits exist as a spectrum and only are characterized through relativistic means. For instance, the mortal realm or “prime material” plane could be used as the benchmark of standard or neutral, but compared to realms below it is a paradise, and compared to the realms above it is a purgatory. As such, objective measurements are useless. Instead, there is only higher or lower. However, these realms do not necessarily have to be cosmically separated from each other; they could be places in the core setting- in the same way in some more mythologically inspired worlds you can climb to heaven or dig to hell, these could apply to these too.
Additionally, each realm exists on a limit, but the limit can also expand infinitely outwards in horizontal motion as well. You might well exist in a high-fantasy medieval world, and then next to it is a bronze age mythology inspired fantasy world- these are different realms, but they exist on the same level of the cosmic chain.
Realms Higher on the cosmic chain to your own are usually heavenly realms, spirit places, or seem to exude both power, magic, and forms of authority or morality on the realm beneath. Examples of these realms to a human perspective would be places like the Celestial spheres, Asgard, the spirit realm, or other realms of enhanced reality.
These places are, in comparison with a lower realm, more beautiful and filled with life. The beings and races within this realm are more powerful by birth then anything from the lesser realm- spirit beasts are more powerful then mortal animals, demigods are greater then the greatest mortal champions, they have no base beings like orcs or humans, but superior creatures like cloud giants or numenoreans. You could imagine the standard fantasy races as being here but just “better” versions, like wood elves in your home setting are more like high elves here, OR these races are totally replaced with greater beings. The default of generic character/mortal/character sort of standard in this setting is just higher then from the main world.
In this realm, food and drink is of another level higher then the middle realm; even a pauper has lavish feasts fit for a King in our place. The same to goes for magic and treasure. The Gods or Kami may trade platinum coins between themselves for standard transactions; gold is too common and lesser of a material. These realms also often contain supernatural metals or materials too that is much higher in strength and purity then anything in realms below; one of these falling to the mortal realm is equivalent to a high level magic item being dropped at random.
To beings of lesser realms; coming here is like a supreme challenge. Everything, from the most basic of threats to the competition from intelligent beings of this realm, become incredibly more difficult as you aren't made of the same stuff. Groups of adventurers of high or name level will be akin to starting adventurers in their home country- imagine dragons or giants being basic level 1 enemies. It would incredibly difficult to carve out a niche in this world- though richly rewarding as even the most basic of treasures or companions would be greater then the greatest in your own world.
Also; beings who come from a lesser dimension to this one may find themselves becoming more powerful over the course of their life from ingesting its food and breathing its air; or through a bit of Lamarckian evolution their offspring would be considered greater. For example, a human mating with a God will produce a demigod, or simply living and being raised in a higher dimension equates to better-then-perfect growth conditions from your home world, resulting in superiority.
Realms Lower then your own on the cosmic chain are dark, shadowy, or entropic places. Lost or fallen spirits tend to end up here, either as punishment or weakness for being unable to stay in a higher realm. You could imagine dying to be like falling down a realm, where as ascension is going up a realm. Many scholars theorize the cosmic chain is not a linear chain but more like a steep funnel; it's incredibly easy to fall down, but very hard to climb up. Examples of lower dimensions include hell dimensions, shadow realms, ghost or veil worlds- something like the Upside Down or Silent Hill's dark world would be examples, or the shadow world/dark world from many games could apply. You could also apply this to fallen worlds too; something like the world of Dark Souls could be thought of as having fallen into a “lower” cosmic state then it used to be.
These places are, in comparison to the neutral middle realm above it, darker and deader. They tend to be more quiet, with less biodiversity and less life- the cosmic order and laws of nature are less important here, which tends to mean more chaos and unreality in a bad way, as opposed to higher realms which are more filled with magic, these realms are more filled with curses and oppressive darkness. They drain energy instead of giving it; much like a negative energy plane. For this reason, the beings that live here are lesser in number, weaker, less intelligent, and less moral or reliable then beings above them. You could imagine “dark” or stunted versions of the mortal races or animals- perhaps just the standard races but with more present mutations or general apathy, or a more pre-modernist view of the ancient world (people were old at 25 year old, everyone was dumb and illiterate and burned witches left and right; that kind of nonsense) but taken straight as how these lesser people actually are. Or the more standard races or animals could be replaced with totally new ones; things like the Tarkatan from Mortal Kombat; not necessarily inferior to beings of the realms above in terms of strength but more savage and less refined or spiritual.
The physical world of this dimension is also lesser then the one above it. Food may rot quicker and silver, if it exists at all, would tarnish quickly though would be unparalleled in its beauty. Most physical items are less sturdy and rot away faster, people are less wealthy regardless of the actual economic systems; Kings and nobles of this realm would live in homes that a standard villager in the core fantasy world would have. In the same way, magic and warfare are similarly different; the most powerful archmage in this setting may only be able to cast 3rd or 4th level spells from the core fantasy world; your apprentices are equivalent to practiced magicians in this world. This could just be because the ambient magic here is weaker then the upper dimensions; though this wouldn't stop a tremendously powerful magic from your world being an unstoppable demigod in equivalence. As for combat; the same applies- the most powerful troops here may have cobbled together armor and weapons and fight about as strong as some goblins or orcs if they're really outstanding.
Exploring this realm from the realm above would be very easy- you would be outclassing everything here with even a modicum of experience. To the people of this realm, your characters would seem incredible beautiful, smart, strong, and powerful with only average or slightly above-average ability scores. You would very much be like characters in a video game where you're worshiped as the chosen one after the tutorial mission! Even a medium level group of adventures from this world would be able to defeat and dominate almost everything here- the greatest monster in this world may be a single ogre or troll in comparison to the weakness of its people. It would be incredibly easy to dominate this world and live as Kings, but everything here would hardly be worth it. The treasure would be much less value then that from your home world; gold may not even exist, the biggest gemstone is just a tiny thing that would go on a wedding band in your world and here it would be the crown jewel of an Emperor.
Staying in this realm for extended periods of time from a higher dimension would be bad for the health. Perhaps its because the realm is higher in quantities of negative energy (or just lacks as much life energy); you'd be eating poor food and breathing poor air, and you'd slowly degrade over time from this, despite your inherent superiority over the beings here. It could also just be the same as the higher realm where only children born here are lessened- in much the same way when a God comes down and has a demigod, they are lesser then the true God. In the same way; a human breeding with a being in this realm may result in a greater then average but lesser to what the human would be in the real world. This feels very similar to how Gods might degrade their bloodlines through mixing with mortals is what happens if you live among these lesser beings from a world lesser in cosmic importance.