The rest of the family
The ACE didn't arrive fully formed. It's the fifth machine in the AC6502 family and it's made of everything the first four taught.
They all still exist, they all still run the same BIOS, and they're all open hardware. If you want to build a 6502 rather than use one, this is where the interesting reading is.
How it went
COB — Computer On a Backplane. The first one. A passive backplane and a card per function: CPU, memory, video, sound, serial, storage, GPIO, clock. Building it a card at a time is how each piece of the architecture got proved, and it's still the clearest way to see how the machine is put together.
DEV — Development Environment Vehicle. A Teensy 4.1 pretending to be a 65C02, so the CPU could be stopped, stepped, and slowed right down. This is where the emulator came from.
VCS — Video Computer System. The first unification: a Main Board carrying CPU, RAM and ROM, an Input Board for keyboards and joysticks, an Output Board for video and sound. A cartridge console: games drop into a slot standing up on the board, the same way they do on an ACE.
KIM — Keypad Input Monitor. A KIM-1 homage built for fun out of parts that already existed. These days it's better thought of as an add-on that turns an ACE into a KIM, which is why it has a chapter in the main guide.
ACE — All-in-one Computer Experience. Everything above, on one board. The machine this guide is about.
What they share
Every one of them runs the same BIOS, with the same BASIC, the same Monitor and the same memory map. A program written on any of them runs on all of them, as long as the hardware it wants is fitted — and the BIOS checks that at startup so nothing crashes when it isn't.
That's the whole reason the family holds together, and it's why the ACE could be built by absorbing the others rather than starting over.