@HellFireKoder thank you so much

XML is just the file format the plane is saved in, it stands for "Extensible Markup Language", and it may-as-well be a normal text file, but it's arranged in a "prettier" way.

You can edit the XML file to make "mods", but these are just "XML mods", and only let you modify properties of stuff already in the game, however there are real mods, too, often called "Unity mods" to separate them from what people normally call mods (the XML edits), and they are made with the Mod Tools package (which comes with Steam version), and Unity (game engine and editor), these mods can do ANYTHING to the game, though modifying how any part of the game works is troublesome, because you may need to replace several other parts for it to work, but it is possible, and they can add all kinds of new stuff (parts, maps, levels, and more).

Because of Apple's policy however, you can NOT get real mods on iOS, because of just that, I would recommend the Steam version, though there are very few real mods out yet, people are working on some mods.
(Android can also use real mods, but... PC can handle more than Android or iOS, unless it's a really old PC or something)

XML are mods, just search modded parts, and no steam is not different, but i don't play on iOS #PCMASTERACE