How to make one personal account for several sites?

Question

CMS

You need to find the best solution to implement the idea:
1. Create several sites
2. So that all sites have one personal account (PC). That is, the user can register / log in to any site, but the personal account is always the same.
3. All sites have a common referral program. That is, if the user is registered in the personal account, then he has a ref. link for any pages of any site in the system. A person who registers on his ref. link, immediately becomes his referral.
4. It should also be possible immediately (or in the future) to make access levels. Depending on the access level, the user can only view certain information.
Connoisseurs, tell me the best solution.

Answer 1

If you have conceived something more or less serious, then it is better to contact a programmer and do it right away, as it should.