RSUs are taxed as income when both vested and released (generally simultaneous). From then forward, gains are capital gains from a basis of the value that was taxed as income.

With respect to lot identification (what shares did you actually sell for tax purposes), within an account, most brokers will let you elect specific lots or FIFO and I believe the IRS allows you to elect average basis. (There’s no particular advantage to making that election, IMO, so I never looked into it but vaguely recall that being the case.)

Across accounts, except for wash sale treatment, the IRS does not assume that when you sold in account B that you were selling shares acquired in account A.