Trump actually carried the county Fargo is in both in 2016 and 2020, but not by wide margins (he carried the state by more than 2 to 1 both times). The only two counties he lost in 2016 and 2020 have Indian reservations. It’s possible that he lost Fargo city and the remainder of the county put him ahead.
I thought Indian Reservations were treated as separate countries. Why do they get to vote in US elections?
Yes, that is the case. Here is the precinct data for 2020 for Cass County:
2020 presidential election results by precinct, Cass County, ND
(blue = good, red = bad, as it should be with all political data)
Fargo hardly "dominates" even the county it's in. much less the entire state like some poster above claimed. Not even close.