[wcrossbow]

You can flash your own firmware which you can inspect. QMK and ZMK are two very popular options.

[dmd]

By "very popular", you mean "as many as 0.0001% of people worldwide use it", though.

[wcrossbow]

That's a gross underestimation. At current world population levels that comes out to be 8000 people. The QMK github repo alone has over 18k stars and almost 40k forks. So yeah very popular!