Airlines categorize pax with special needs into 3 categories: WCHR, WCHS, WCHC, depending on the assistance required. Normally you request this at the time of booking.

Bigger airports have electric carts and wheel chairs, smaller have only wheel chairs.

When checking in the agents sees that your PNR is flagged and calls the cart/wheel chair to the counter. At arrival the cart is waiting for you at the gate or the wheel chair at the cabin door.

Some airports are flexible, but requesting ahead ensures the service which is free by the way. In FRA I never ever tip. The service is below lousy. Always understaffed and delayed. Total different picture at Thai airports: Only wheel chairs, even at BKK and the service is very personal, prompt, efficient and yes, fun. The boys get a tip almost every time.