It's a very broad question.

On tech, it is believed there was an unwritten agreement to share even nuclear tech from Manhattan, after UK provided some scientists, both British and refugee. The agreement seems to have been void after Roosevelt's death.

Obviously, pre America's entry, Roosevelt was well known to be very pro British while US remained ostensibly neutral, with Lend Lease, the Atlantic Charter, etc.

However, once allies, and that great goal of winning the war achievable, the two powers could afford to be more divergent and so disagree about their aims and priorities. The shift in dynamic: UK about to lose great power status and its empire, and the USA on the threshold of becoming a superpower, also affected relations.