Which imho is one of the things that make web development nowadays obnoxiously difficult - there are tons of frameworks and stuff trying to reinvent the wheel. Keeping up to date with vanilla web development alone is pretty hard, I don't know how people can keep up doing React/Django/SQL/AWS/whatever. It's too much.
Part of a reasonable answer is that you do it with whatever toolset you’re familiar with. It’s also reasonable, if you used a signed cookie system that the cookie verification module is about 50 LoC so you could use an auth system written in (say) PHP to authenticate a node.js or .NET application.
In 2000 I had been watching Yahoo buy up lots of internet properties and… integrate them with their auth system. I had a vision that you could pick “best of breed” open source and other software, plug it into your auth system and make your own “portal” web site. I had some success developing sites this way but it never caught on and I still don’t understand why.
