Is it possible to use NodeBB with its REST Write API headless with an own UI with own design?
Is the complete needed functionality for a forum available via the REST Write API?

@pitaj https://github.com/julianlam/nodebb-plugin-session-sharing explains to to use this plugin and sesssion sharing to automatically login. SSO and OAuth2 is not needed.

There is also a forum entry https://community.nodebb.org/topic/11372/sso-single-sign-on-opportunities which suggest to use SSO and OAuth2 in order to share sessions.

"... The recommended method of sharing sessions between two separate and distinct applications is through OAuth2. ..."

I am a little bit confused. Who is wrong?
Do I misunderstand something?

I have to implement a web app. This web app will have a login for the user. Internally it will have an own user account database and an access rights management.
In addition I have to integrate a community forum like nodebb. It is the goal to switch to the forum with automatic login (single sign in SSO). I think nodebb has it's own user account database.

How to synchronize both user account databases and realize SSO?