To add to what @denton@lemm.ee said, you can also buy kits from sites like Keebio and Novelkeys. There is a Etsy store call BeeKeebs I will always suggest because Leo is a really cool dude and offers a lot of kits for stuff that you'd otherwise have to part yourself.
I print my own PCBs and aim for boards that don't require diodes. I have most of my boards with Choc Sunsets which are aftermarket custom low profile switches form loweprokb.ca. All my boards run on a fork of QMK called Vial and the hobby gets weird from there. If you are on discord there is a meckkeys server that has a ton of info.
For how custom they can be: Look up Ben Vallack's Piano2. It's an 18 key board that he uses for everyday use including writing code. QMK and it's forks are extremely powerful pieces of software that still blows my mind and I've been using it for 3 years.