How to Realize Single Sign-on by Separating Front and Back End?

  question

Suppose api interface is api.a.com
The login operation was completed in login.a.com
When visiting b.com and c.com
After the user login, the backend will return a token to identify the user
Neither cookie nor localStorage can be accessed across domains. how can I get the user id and save the user status across sites?
After logging in like Taobao, I was already logged in when I visited Tmall.
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I saw it this morning.From the Nuggets