It's actually pretty simple.

Assume you have access to no lounge, unless --

If you are a member of the AAdmiral's club, you can use the AAdmiral's club.

If you are PLT or EXP, and traveling internationally, you have access to busines class sounges, a.k.a. the AAdmiral's clubs.

If you are EXP and traveling internationally you have access to first class lounges, a.k.a. the Flagship Lounges (currently only in ORD/DFW/MIA).

I have been told conflicting things about lounge access for trans-con flagship service. Sometimes first class passengers get access, and sometimes they don't. Business class passengers never do.

That's all she wrote.