You download whatever TS4 item you want and it goes into your Mods folder. You should probably read the Sims 4 information in the Sims wiki for a full explanation.
It is easy to get rid of custom content or mods. You just delete the unwanted ones from your Mods folder or delete the whole Mods folder if you want to get rid of everything - the game will generate a new one.
Maxis definitely does allow custom content and mods. They would probably urge caution but that is not a bad thing.
Just be aware that many mods and some custom content have been broken from the most recent patch and creators will need to update them to get them to work as they should in your game.
