Depends on what it is. If it’s meant to be mainly used in the shell I will usually use whatever language the shell uses. Anything that’s run automatically usually gets done in bash for compatibility, and stuff more complicated than a few loops and some piping gets done in something easier to work with like python.
For getting better at terminal I would suggest looking into alternative shells. Bash is fine, but it’s not exactly user friendly by default. Something with more robust auto completion like zsh or saner defaults like fish could make the learning experience easier. You can always come back to bash later.
For understanding how the OS works I would start by reading about the file system layout, then look into the init process.
I got to the point I am at through a series of projects of increasing complexity. First I ran a web server on my machine to copy files over the network. Then I used a spare PC to make a simple SMB server. Later I made it into a HTPC pirate box, for streaming stuff downloaded off Usenet to my Xbox. At some point I ran a minecraft server (before docker came along and trivialized this), and got into a bunch of sysadmin and programming stuff and that’s pretty much it.
Seems convoluted