This depends on the model of the scooter and seat that you’ve chosen.
There are two main ways to attach a seat. The first is by screwing it into pre-drilled holes in the scooter’s deck. The other is by attaching it using a clamp mechanism. Whenever you buy a seat for your scooter, you should make sure that it is compatible.
It’s a similar story when you want to remove a seat. However, plenty of brands have designed their seats so that you don’t have to unscrew the whole thing when you want to take it off. Instead, a quick-release lever will allow you to remove the seat post and saddle, leaving the base plate attached so you can easily put it back on when needed.