The pickup for each rental varies. When you select your pickup/return location you should read the description for the location you selected. For extra clarification, ask the agent handing your the keys how you should return the vehicle.
Usually if an airport pickup is selected, an agent will meet you at the airport arrival hall and drive you to their offices in Keflavík, some rentals however have a rotating shuttle bus which you can hop on to take you to their offices nearby. Other rentals have offices inside the airport.
The protocol differs on guesthouses/hotels as well. Some rentals pick you up and deliver the car to you at your desired location, some rentals pick you up from your desired location and drive you to their offices where you receive the car. Returning the car at these locations also differs. Some rentals allow you to leave the car at the guesthouse/hotel, while others require you to drive to their offices, hand over the keys and they'll drive you back to your guesthouse/hotel.