UI stands for "User Interface." "SIM" stands for "Simulation." A UI mod affects only your computer. It affects how you interact with the game. A SIM mod can make changes to the game for other players. A UI mod can change how information is displayed to you (for example, it can change the scoreboard) and it can turn things on and off for you, like it could turn your radars off to save power. A SIM mod can make much more serious changes. A SIM mod can do anything a UI mod can do, and it can do a lot more. For example, a SIM mod could change the amount of HP that a tank has. Or it could make the tank into a flying unit and give it nuclear missiles. A SIM mod could change how the scoreboard looks to every player.
A SIM mod makes the game unrated, so if people choose to play with you, it doesn't really "matter" whether the game is fair. If you think it's unfair, just don't play with that person, and you won't lose any rating if you do.
When you play a custom game, every player gets their own UI mods, but the SIM mods are supposed to be identical so that the game stays synchronized. That is because when you run the game for multiple players, each player's computer is keeping track of the entire state of the game. It would break the game if, on your computer, a tank had 300 hp, and on your teammate's computer, the tank only had 250hp. Errors like that are called "desyncs" because the game stops being synchronized.
It is true that if one player has different UI mods than another player, it could give them some advantage. Every player has the ability to choose the UI mods they want to use.
UI mods don't give a huge advantage. I know because I've played quite a few games with no UI mods, because I turned them all off and forgot to turn them back on, I didn't play that much worse than normal. Don't worry too much if someone has "better" UI mods than you. It's a good idea to try out some popular UI mods and decide if you want to use them but you shouldn't worry that people are getting a big advantage over you. Some people have put together collections of their favorite UI mods. For example:
https://forums.faforever.com/viewtopic.php?t=19207
https://forum.faforever.com/topic/1186/ui-mod-guide-for-the-improving-player
Not all maps support the ability to remove mass points. Basically, only "adaptive" maps will support that, and you can tell whether a map is "adaptive" by whether the name of the map starts with the word "adaptive." If the map you are playing has "adaptive" in its name, it probably has features like that. When you are in the lobby you can change spawns by right-clicking on them (cycle between "closed" and "closed-spawn mex") and you can change other map-specific options in the lobby's options menu.