This is a fun, and I enjoyed the "CRT Dude" video because I resonate with that need to understand :-). One of the things I learned during that era was that there were a lot of computer makers but relatively few factories in China that were making things to assemble them. Because it was simpler to take the sheet metal work that was already designed and being made for brand 'x' and then differentiate on the case molding and electronics, a lot of the mechanical components were "re-used" (the factory can make 1000 or 10,000 with the tooling and the more they make the easier to amortize the tooling costs so the cheaper they can offer them).
I suspect when a company spec'd out a new design and then got all the tooling done, unless they explicit language in their contract about selling stuff made with the same tooling to others, the factories could pitch "we will do the basic case with no NRE[1]" and that was a bargain. As a result a lot of things ended up being "magically" similar in those days.
[1] NRE = Non-Recoverable Engineering which is the cost label for the engineering work to build the jigs and tooling that the factory will use to make the parts you want. Example an injected molded switch cover might cost $10,000 in NRE to make the molds that can produce 10 switch covers each and be used up to a 10,000 times. Then if you make 10,000 switch covers, you have used the mold one 1000 times and used up 10% of its lifetime. Cost of the plastic plus $1,000 (the 1/10th of the cost of the mold) are the real cost of those switch covers.
I suspect a big factor was PSU design. Even large OEMs contracted it out, and there probably reached a point where manufacturers started to offer catalog items in the 200-400 watt range where the primary size constraint was the 80mm exhaust fan. Once that's the constraint, it's going to look a lot like LPX, and you get a reinforcement factor around specific dimensions when vendors say "If we use the exact same screw positions we can compete as a drop-in replacement"
It's interesting that PCs adopted the LPX PSU but not the cases and motherboards. I had always seen LPX described as a bit more proprietary than (baby-)AT designs-- you could fit anyone's mainboard in a generic AT case, but even a HP "LPX" mainboard and riser card might not fit in a Packard Bell case.
In the 1990s none of that stuff was made in mainland China, not even the cases. A great deal was made in Taiwan. Even more was still made in the USA.
Personally I remember AST -- a PC clone vendor who specialized in vertical integration, all in the USA. USA-made accessory cards, fit into USA-made motherboards, fit into USA-made cases, run by USA-made power supplies.
All this dried up and blew away, including the parent company, as both mainland and Taiwan capacity improved and prices fell, but it was not that long ago that you could literally build an entire PC with American components. 25 years, not 50 or 70.
These days the majority of computers are designed and manufactured by a small number of OEMs too.