I don’t know how to answer your post exactly, but in the spirit of trying to help, I will try. The DIY mowers are machines many of us have built to be fun projects as well as useful tools to mow large acreage in locations where it is safe to operate them. They are complex pieces of electro-mechanical hardware that generally require many months to build, and requires the builder to have many skill sets (which is what makes them fun and involves a lot of learning). These machines can be programmed via your laptop to go out and mow a defined complex area and then return back to base (much like a drone would do). In fact, they use the same onboard computer/navigational systems that a drone uses.
Every mower is built differently! A key component is the base mower. Some people build them from scratch and some people start out with a commercially built mower and convert them over. They can also be electric or gas, which makes for an infinite number of possibilities. People think differently and choose many different equipment options to put on these mowers based on cost, and experience. Every machine is as different as the people that build them.
Many of the questions you ask can be answered from reading the
https://ardupilot.org/rover/index.html
The posts in this thread and a few others also have good information.
https://discuss.ardupilot.org/t/best-gear-for-starting-ardupilot-mower/
https://discuss.ardupilot.org/t/new-build-dev-prod-setup-toro-60-zero-turn/
https://discuss.ardupilot.org/t/yuris-mower-updates/
Note, in general we try to keep most of the posts public, so other interested people might benefit.