Would you not want to sell stock you have held for some time to take advantage of lower capital gains - or do the US tax authorities assume you are selling older stock first?
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.
e.g. if your RSUs are valued at $1000 when they vest and you sell it a few minutes later and the value is now $1005 you'll pay regular income taxes on $1000 and have a $5 capital gain (i.e. when you file your taxes the cost basis for the holding are $1000, not $0)
You surly don't pay income tax on the gain of already owned stock but CGT.
Back in the day 2000's I did own stock that was worth over 1,000,000 certainly wouldn't have had to have paid income tax if we had been bought out at point - but that was in the UK