The easiest and lowest difficulty level setting in Civilization 6 is “Settler”. At this level together with “Chieftain” and “Warlord” levels, the computer player is penalized with -1 combat strength. In turn, the human player receives combat strength and experience bonuses along with extra gold from barb camps.
Prince level of difficulty is the default, and it is where the the human player loses all perks and the computer player begins to receive various bonuses.
Starting from minor bonuses to science, culture, faith; production, gold, techs and civics – on Prince to extra starting units on King and higher difficulties – culminating in 2 extra settlers, 4 extra warriors and 2 extra builders on the highest level of Deity.
In short, the computer player does not seem to possess much Artificial Intelligence, but rather given a head start to make it challenging.