One of the great things one can learn from SQLite is the degree to which they unit (and integration) test their source code. It's honestly the best unit test document I have read in my career to date: https://www.sqlite.org/testing.html.
IIRC, some company wanted to use SQLite on an airplane, so they paid the devs enough to bring the test suite up FAA standards. IIRC, they have code coverage of every machine instruction.
That was my business plan: Do the intense testing required for avionics, then sell the test cases to aviation manufacturers. That plan didn't work out - I've never sold the tests to any aviation manufacturer; not one. But the TH3 test harness has had side benefits that I did not anticipate, not the least of which is that it allows us to maintain a complex code base that is run on billions of devices with just a few developers.
Yep, I think (with 90% certainty) that Richard Hipp, the creator of SQLite, mentioned this in a Youtube Talk, but sadly I can't recall which one. :(