They are making it up as they go and the plan was not wholesome.
Ideally they would just be able to isolate players and contact trace, etc., and fill in rosters from the 60 man pool. That's the whole point of the 30 man rosters and 60 man pools.
BUT failures in player discipline, testing accuracy, testing turnaround, and complications from local and state policies, have ruined the plans.
Baseball's hope for how it all could or would work seems to have been too Utopian.
NHL and NBA have taken more realistic approaches with their bubbles. MLB really should have bubbles in a few select places, possible with Toronto being one.