As others have said, you'll have to enter the US as there is no international transit option at MIA or most US airports.

I just had a little over 4 hours in MIA last week and spent time in the Centurion Lounge and D30 Admirals Club which are the only two options open at the moment. If you have the option to go to the Centurion Lounge, don't waste your time in the Admirals Club. I had a great experience in the Centurion Lounge and essentially had the lounge to myself for two of the hours I was there. When I first arrived around 11am there were people there, but almost everyone quickly emptied out. Great service at my seat too without even having to get up, though this may or may not happen depending on how busy it is.

Keep in mind shower facilities at both lounges remain closed, so plan accordingly if you were thinking of showering between flights.