User authentication vs Login?

Hey everyone,

I am a fairly new SWE, and never worked on anything like the login functionality to a website or application. Its something that I have been thinking a lot about, how are these systems usually built? I have been reading online that people have moved away from hashing and salting passwords and storing them to user authentication, how does this work to show dynamic content? For example, when I log into Reddit, Reddit knows my subreddits and these are different from your subreddits. Does anyone have any good material that could explain this for me?