I want to develop a "common" login page for all different websites that our organization offers. That is user should be able to log in only with a single username and password and be able to access all different organizational sites allowed to him.

I can develop a React login page. But what is the "standard or common" approach followed in the Industry to make this login page common across different sites? Should I do some kind of redirection if the user is not logged in and is trying to access a webpage requiring being logged in? If yes, how? If not, what else?

Some more details

User can hit different organizational site based on his role. For example, organization will have separate portal for international candidates applying for admission to one of the course and hence will have to use username-password. Existing students / professors / staff have separate portals and may opt for LDAP login or Google login or username-password.

I feel having single login page for all of them will be good. Clicking login on these different portals should take all of them to login page which should allow all these different types of login.

Or am wrong with having single login page?