If you go read what SWBKCB has helpfully posted, it’s right there. Document entered force 25 March 2021 so post Brexit. Article 2(1)(c)(i) and 2(1)(c)(ii) are the bits you need to read.
US airlines have full traffic rights for point behind USA-USA-UK and onwards if they wish, so fifth freedom rights. So Fiji-LAX-MAN-FRA would be possible (for instance). UK airlines have rights to fly XXX-UK-USA-YYY - so could fly Kuwait-MAN-JFK-YVR for example.
The only thing carved out is domestic rights in each other’s countries (cabotage) which is pretty normal.
The agreement also supersedes other agreements including the 1977 agreement signed at Bermuda which is commonly referred to as Bermuda II, and the revisions to that which followed over the years.
There is no reason why a US carrier wanting to fly to Manchester could not start doing so tomorrow.