Searching HN for such a term often gets me at least some of the way there.

It depends on what you mean. Learning ISA's is easy. Learn a few different assembly languages, write a disassembler and maybe an assembler for one of them, which will require you to study the documentation. Intel has a ton. There's a ton on RISC. I assume the same is true for ARM, PowerPC and older simpler architectures. Maybe start with the 6501 then an 8086. To go deeper, there are plenty of CS books and video courses online that cover those subjects more theoretically.

Designing and manufacturing? How deep do you want to go? If you just want to know the basics then YouTube has everything you need. If you want to be involved, then you'll want to get an education in that specifically. It's not something you can just learn at home.