I’ve been wanting to get into self hosting for a long time and messed about a bit with setting up a nextcloud. But while I am reasonably well versed in linux and programming all things to do with the internet are quite confusing and mostly after reading blogposts and the like I am feel like I am missing so much knowldedge. From why people use traefik, how and why to use docker containers, what settings to change on the router, how to connect and setup the domain, and probably most importantly how to do this safely. Is there a good book or site that explains these more basic things?
No, there is not one resource for all of these things. Any guide that takes you end to end setting up one service is going to omit a lot of important context. I would research each step individually.