If I get something for this Fortnite season, is that only for this season or will something stay in the next one?
If you get the Battlepass is only for this season afterwards you can buy a new one. You keep the things from the Battlepass. If you buy something from the shop, you will keep it forever (skins, chopping, paintwork and backpacks and dances)
If you mean skins or dances then they stay forever and if you mean the Battlepass then only for the season.