If one is interested in how internal PC registers work, than these build series do the classic eeprom microcode based CPU builds. Fabian's series is highly accessible, and builds a python based assembler from scratch. James series ends with a simple game design.

Cheers, =3

"Build a Superscalar CPU" (Fabian Schuiki)

https://www.youtube.com/watch?v=bwjMLyBU4RU&list=PLyR4neQXqQ...

https://github.com/fabianschuiki/superscalar-cpu

"Making an 8 Bit pipelined CPU" (James Sharman)

https://www.youtube.com/watch?v=3iHag4k4yEg&list=PLFhc0MFC8M...