As an ex Practice Manager, I can tell you that most GPs don't receive a salary. Their income is made from claiming for every service their provide. They get a certain amount per patient, (not much!) and then claim for anything the practice does. The vaccinations may be done by nurses or GPs, the claim goes in, and the money comes into the practice pot. Same for cervical smears, treatments for diabetics, asthmatics, running clinics, etc. A GP going into a nursing home to do vaccinations will take him/her away from other services, it is right that they should get paid.
Salaries for staff, and non-partner GPs are paid out of the pot, also maintenance of the buildings, IT equipment and support for it, practice drugs for the emergency bags, and all equipment for the consulting rooms and office spaces.
These are large independent businesses, they have an accountant which advises the GPs, after all the other expenses have been paid out, what is left over and how much they can take as 'drawings' which is their income. The senior partner gets the most of course!