Also software like SQLite:

" SQLite is built using a DO-178B-inspired process. The testing standards for SQLite are among the highest for commercial software.

SQLite is open-source but it is not open-contribution. All the code in SQLite is written by a small team of experts. The project does not accept "pull requests" or patches from anonymous passers-by on the internet. "

https://sqlite.org/hirely.html

https://sqlite.org/testing.html