If you are looking for some less garish designs for cases the Fractal Design North is one great looking option, and also some of the mini-itx form factor cases look really nice too, not too far off from some of the ones you mentioned.
That said, you absolutely can add a graphics card to some of the corporate machines, but some are much harder than others. I have not done one recently but honestly I would look for someone else’s build and replicate it as exactly as possible. I have had weird issues with corporate gear before around connectors, weird pinouts, very limited PSUs, and strange limitations on the PCIe port supporting some cards but not others (HP and a generic network card crashing my system randomly). If you find that someone else has done that specific box with a specific card then you should be ok, but if you have to deviate try to make sure that you match closely what they had, such as whether the graphics card has extra power or is PCIe only or if they have to use one of the corporate style graphics cards because gaming cards won’t work.
I don’t know about videos but having a look at the OSI model is a good way to start. It covers the abstract framework for packetizing data including things like the distinction between hardware and software, envelope, encryption, application layer stuff, the whole shebang. The cool thing is by going hardware, network, application you can see where responsibility are and it helps you understand where things can go wrong.
If you are interested there are plenty of CCNA style courses available on the internet, licit and otherwise, and they go into more depth, and the same applies to RHCE/RHCSA material. The training for certifications like that covers what you want to know but also puts it in context, and again licit and otherwise sources are available.