Fifa 18 squad building challenges

Hello I've started playing Ultimate team only really to earn a few PS trophies to be honest and see how high I can get my trophy percentage on Fifa 18. I'm a little confused about the squad building challenges. Apparently you lose the players you currently have if you do it. Some people say you lose every single one of your squad players but others say you would only lose the ones you put in the squad building challenges.

So for example if I put someone like Charlie Austin, Drinkwater etc in it does that mean I would lose those 2 but still keep the likes of Henry, Sanchez, Ferdinand etc? or does it literally mean I would lose every single one of them?