When a player enters the "portal" does he/she immediately relinquish their scholarship?

No. A lot of times nothing happens at all. They withdraw from the portal and carry on as they were.

Thanks

School can indeed remove them, yes.
LSU has chosen not to do that with Arik Gilbert for example hoping to keep that bridge traversable.

Many other players lose all support entirely. Case by case basis and a major pitfall many don't expect.

Kind of like the players that listen to those hoping to jump on a financial bandwagon of leave school early and high draft pick only to “realize” the per diem train and a 15k UDFA “signing bonus” are the real deal many get.