Multi-site Single Sign-On solution!

The new task just took over has a big head! Currently, there are four sites: Discuz for A, wordpress for B, C, and D for independent development projects. Each of the four sites has A certain number of users. Now we need to integrate single-point logon, these four sites are all connected in succession, so the current integration feels like... the new task just took over has a big head!
Currently, there are four sites: Discuz for A, wordpress for B, and C and D for independent development projects,
Each of the four sites has a certain number of users. Now we need to integrate single-point logon. These four sites are only available on a land-based basis, so it is a bit difficult to integrate them.
Is there any solution for reference without using Discuz's Ucenter?

If a user has registered a user on Site B and Site C respectively, they both use the same email account. What should I do if I want to log on to site D?

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The new task just took over has a big head!
Currently, there are four sites: Discuz for A, wordpress for B, and C and D for independent development projects,
Each of the four sites has a certain number of users. Now we need to integrate single-point logon. These four sites are only available on a land-based basis, so it is a bit difficult to integrate them.
Is there any solution for reference without using Discuz's Ucenter?

If a user has registered a user on Site B and Site C respectively, they both use the same email account. What should I do if I want to log on to site D?

Ucenter is one of the best solutions.
Of course, it is also possible to share cookie_id with the parent domain name under the same domain name.

Website A provides OAuth2.0
Other websites use A third-party website A to log on.

I don't know if this idea works?
First, create a new user table for the user tables of the four sites. Then, check whether there are any users in the table during the query. If yes, log on directly. If no, go to the four sub-stations and query them one by one. Then, write the user information to the new user table. Of course, you can also use a script to directly import all the user tables of four sub-stations to this table at one time. Personal opinion, for reference only :)

The user password encryption methods of various systems should be considered;
You only need to integrate the membership data of all systems before you can perform single-point logon.

Manage your entire membership system. All Station logins go to this system to call the api.

For reference ..

Isn't ucenter a member management system ??? Why do you step on-_-... please advise?