Honestly, what most people lack is foundational networking. How a TCP packet is constructed and routed, for example. The difference between routing and switching. How a TCP connection is built and destroyed. Stuff like that. And a basic knowledge of how a DNS request is made and resolved. And DHCP.
There are a heap of general "Linux Administration" courses which will patch a lot of holes in the knowledge of almost all self-taught self hosters. I'd been using Linux for a while but didn't know you could tab to complete file names in commands till I learned it on Udemy ¯_(ツ)_/¯