SuperTokens is an open source authentication solution offering features like: Different types of login: Email / password, Passwordless (OTP or Magic link based).

SuperTokens.com

So if you have multiple frontends that talk to the same API layer, then:

(frontend1, frontend2.. frontendN) <--> API Layer <--> SuperTokens core

If you have multiple API layers per frontend:

(frontend1 <--> API layer1, frontend2 <--> API layer 2.. frontendN <--> API layer 3)  <--> SuperTokens core. This will share the same users across all frontends

Finally, you can run one core per frontend and backend. This will essentially create different users per app..