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Hopefully, you have Global Entry, so you don’t have to clear customs and immigration.
United Domestic flies into Terminal 3 (concourse F).
The international terminal is next door and contains concourses G and A.
Concourses F and G are actually connected airside so if you’re going between those two it’s dead simple.
If you’re United the whole way it will quite likely be G – that’s where the Polaris Lounge is.
If you are leaving out of A you will have to leave security and go through again at that concourse.