its a good hobby but i doubt companies hire, many computer desktop building are sole traders that operate online.
I recently worked for an IT company which ordered custom built computers from a vendor, but still required SSDs to be imaged and replaced, also the occasional GPU, HDD and RAM stick being replaced. But yes, Desktop Computing building isn't really a career you can work off, you could join a computer shop I suppose and work for them for a bit, but otherwise a degree (or diploma for that fact) won't help in this area, because none of them focus on 'Computer Building'; it's sort of a hobby that's learnt on the side. It's pretty easy too, no different to advanced Lego blocks imo.
The next step up, which is pretty huge, would be doing Computer Engineering (or Computer Science/Software Engineering) to understand how the low-level chipsets are designed and created and why so. But this involves a lot of intense maths and physics and isn't for the faint hearted.
I choose to study IT and ended up in an internship which coincidentally let me 'semi build/repair' computers, having done this at home many times, there's not much more to learn, other than cable management techniques and maybe tinkering with liquid cooling/case modding/overclocking, which businesses couldn't care less about, unless you're a dedicated internet cafe operator...?
Lets just say you'll easily get bored if that's all you set yourself up to do in your working life, but it's a good place to start (More IT students should know this stuff back to front, but most just don't have the interest or time), gets you familiar with all sorts of PC issues; software, hardware and networking without even picking up a book.
