you’ll have access to any *G lounge - basically, those are pretty much the lounges run by *A airlines (there are some nuances - for example, generally, LH runs lounges for business class pax which are just for those booked in J class, and separate Honors lounges which are generally their *G lounges. Contract lounges are going to be based on carrier you are flying, and their allowances.

for contract lounges, you’ll generally get an invite card at check in, ideally, the agent will do this upfront if you are eligible, or if not, you can always ask. Depending on the contract, it may or may not include all *G lounge- Air China may only contract for their elite/biz class pax, and not all *G, or they may contract for all *G. When I was *G and based in BOM, for example, when I was flying TK, UA, SQ or others, I’d generally receive an invite at check in (without asking) that I could use for myself and a guest to enter the BOM lounge (there’s really only one) - in that case, it was open to any elite flying on a *A carrier.