~Well you know how some mods say they require ArchiveInvalidation? How come I don't use it but the mods that "require" it still work on my game? All I do is open it up on Launcher, Check, Load Loose Files, and Play. Also I have NVSE.
And also, what advantages would FOMM bring? Does it somehow make the game laggier/ less stable?
Anyways, just curious

Archive invalidation is generally only needed for texture replacers. Basically, sometimes, the game won't load the texture replacers you add, and instead just uses the default textures. I'm sure someone with more technical knowledge than me could explain the cause behind this issue better than I can, but I know it was around since Oblivion. So if you're not using any texture replacers, you shouldn't need it, but if you are and the retextured objects still use the default textures, then you should use some form of archive invalidation.

As to Fallout Mod Manager, if you plan on using a significant amount of mods, I'd suggest using it, it's helpful for doing exactly what it's name would imply. Primarily I use it for modying my load order myself, but that alone can be pretty useful, and it shouldn't effect your framerate when playing the game since it doesn't need to be running during the gameplay, you can use it to make whatever changes you want when outside the game and then close it before starting it.

Thanks for a detailed response. Can you please explain what you mean by "make whatever changes you want when outside the game and then close it before starting it"? lol

He means that you don't need to start the game from FOMM in order to use FOMM's features.

Yes, yes you do. All texture mods require AII and FOMM is just incredibly helpfull, there's no reason not to use them both.