Other commenters have a good bunch of resources on what to do, but if you're really interested in understanding the fundamentals imo there's no better way than to RTFM. Sometimes the manual will have things you don't understand, and then you'll have to google that thing.
For networking I highly recommend the RHEL docs (https://access.redhat.com/documentation/en-us/red_hat_enterp...). There's a big chunk that's useless to you (infiniband, etc), but the basics of the TCP/IP stack is really good to know. It looks like ubuntu has some introductory material too at (https://ubuntu.com/server/docs/network-introduction) with links to more in-depth resources.
For the rest of linux, again I highly recommend the redhat docs here https://access.redhat.com/documentation/en-us/red_hat_enterp.... They're very will written and comprehensive, so feel free to skip all the stuff that you don't care about (printers, SElinux, etc).