Where can I learn about Authentication and setting up Login/ Register page for my SAAS with MERN stack?

I've seen some examples on github but they are mostly using Redux or some other state management, I just want to use something simple with Context API of React and my Mongo DB. Is there a simple way to make this?

I've read somewhere that Firebase has a nice API for that, is there someone which can explain this to me a bit better?

Thanks!