You get access to all *A lounges with a same-day boarding pass, however this would be at the departure airport and you usually don't have access after landing.

In the US you only have access if you have a same-day *A international flight. Same-day connecting flights to/from the international flight would also give you access at relevant airports as long as you show them your international boarding pass.
For any domestic travels (without a same-day international connection) you will need to buy a lounge membership.