Clarification on expansion draft in regards to free agency

I have been a bit confused with what’s going on with the expansion draft since majority of players are free agents. If a free agent is drafted, wouldn’t they just be able to sign with another team (or return to their prior team)? Does that mean teams are able to make really good players available knowing that they’re just gonna sign with them again? I’m so confused on how this is working with all of the free agents and how the expansion teams can ensure the players they draft stay with them.