[0] https://qmk.fm/ [1] https://github.com/qmk/qmk_firmware/tree/master/keyboards/bp...
https://www.1upkeyboards.com/shop/controllers/usb-to-usb-con...
I have worked on Teensy 3.6 support for QMK, and plan to work on Teensy 4.x support as well.
Basically if you want to use this on your "standard" keyboards bought off the shelf, you have to either:
1. Open your keyboard, replace the keyboard controller with something that you can program via soldering etc.
or 2. Pray your keyboard doesn't use a mask ROM to store its firmware, and it has a powerful enough MCU as keyboard controller, then figure out how to reflash it and port QMK to it blindly.
Then there's Ergodox EZ, a company which makes the eponymous ortholinear split key (my daily driver), as well as a 40% keyboard called the Planck, and a new next-gen ergo keyboard called the Moonlander. These are designed from the get go to use layers, where each layer assigns its own meaning to keys, and that's where QMK gets really powerful.
There's a whole ecosystem of kits and group buys if you're looking to make a hobby out of it, but if you just want a keyboard you can program, these have you covered.
I've ordered two Redox from them by now (for work & home) and am very satisfied with the product so far.