Because 'extends' really means 'is assignable to'.
I feel like most of the questions from the post might be answered fairly easily by using reasoning of 'is assignable to'.
Things assignable to A&B are things assignable to both A and B.
Things assignable to A|B are things assignable to A or to B.
'never' means a type that nothing is assignable to and is assignable to everything (bit weird because it usually doesn't have any values but you can create value of type never with the use of 'as').
'unknown' means a type that everything can be assigned to but it's not assignable to anything (except itself and any).
'any' means a type that everything is assignable to and that is assignable to everything (except never).
Ok. Ok. {} types is bit weird way to denote interfaces. :-)