I've stayed at several AirBNBs. Probably the most common, at least for short-term stays, has been that they give you a PIN code you punch into some kind of electronic deadbolt. Whether they rotate the PINs or not, I don't know, but it means you (the client) never hold a physical key and the landlord has it easy to remove you if/when they want to do so.

For longer-term stays, I've just been given a physical key.

Once you go with something much smarter than "punch in a four digit PIN", you'll be dealing with tech support for your tenants. That doesn't seem like fun.