Flying in Europe, it is about a 50/50 chance as to whether you get on the plane from a jet bridge or you take a bus to the plane parked on the tarmac. In contrast, most US airports have jet bridges, even when the plane is small unless it is a very small airport.
Why?
My local airport is exclusively serviced by DH Dash8, so it obviously doesn’t have a jet bridge.
However, my regional airport (ENGM) has all sizes, and the smaller ones have their own area at the very end of terminal A so that passengers can walk to the smaller aircraft out of one out of 6 boarding gates, across clearly marked paths on the apron, and onto the smaller aircrafts.