I think generally, they will come with emulators for popular systems pre-installed so if you are just looking to play common stuff it should either be ready or easy to make ready. Roms are often just drag and drop. So long as you have a reader for whatever card type it uses (Micro SD usually) you should be good. I think some will also let you see the files on the card if you hook up the system to your computer via usb and some won’t.
Installing your own emulators onto these things is varying levels of hassle, so hopefully you’re good with whatever emulators come with it.
Some of them come with roms included as well.
This is not especially useful. It is a way to quickly test it out as soon as you get it and see general performance, but you’re probably going to want to just remove and replace all the roms for any systems you care about.
Reasons you probably don’t want to just go with pre-installed games if whatever you get happens to come with them,
-They come with numbered files which takes them out of alphabetical order.
-They often have English translated titles based on the Chinese title, so they are often misnamed.
-They are not all going to be in your preferred language of choice. (Randomly English/Japanese/Chinese versions)
-They include a lot of hacked roms that are not labelled as such, so you have no way of knowing if you are playing the real game if you aren’t familiar with it, and if the hack is subtle enough, it might take a while to notice even if you are familiar with it.
-They are a random selection of games, instead of your selection.
These are all reasons you probably don’t want to just go with pre-installed games if whatever you get happens to come with them, but some of these might also be reasons why you would want to depending on your character.