Login with FB, google and apple only

[david (David Taylor)]

@kforr

(I don’t want to allow people register with email address)
1st question is if this is possible?

Yep, you need to disable the enable local logins site setting

@kforr

2nd question is: if I only want facebook, google and apple ID, which one should I follow?

You should follow the Facebook, Google and Apple instructions

@kforr

3rd question is: if I want to open up registration using email address in the future, is it ok? and what should I do?

Yes, you can just turn enable local logins back on

@kforr

How can Discourse read my database?

If you want authentication to happen on your own site, you need to choose a protocol to use. The most common is OAuth2 or OpenID Connect. You could also use DiscourseConnect - Official Single-Sign-On for Discourse (sso). This will all require significant development work on your app to get things working.