It's up to the school.

If a player quits the team, a school can cancel his scholarship immediately. However, when a player asks to be put in the portal, the school can't cancel his scholarship until the end of the semester. The difference being one can still be on a team while in the portal. You'll see a lot of players entering the portal right after the spring semester starts for this reason; they know they'll have their scholarship for the rest of the year. If they enter the portal in the fall, there's no guarantee there will be a scholarship for them in the spring.

As to whether or not they'll have a scholarship the next academic year if they come back, that kind of depends on the sport and the situation. Basketball and football probably has less wiggle room, whereas a sport like track or softball might bring someone back.

A lot of players in the portal are there because they don't have a scholarship waiting for them at their old school. Either they were nonscholarship to begin with or their coach told them they weren't going to have their scholarship renewed at the end of the year already.