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?
The last time I had to disembark a flight on the tarmac instead of via jet bridge, it was still a switch-back ramp instead of stairs, and was fully ADA compliant.