SQLite is open for contributions, as long as you "submit an affidavit dedicating your contribution into the public domain" (https://www.sqlite.org/copyright.html). It's not "Open-Contribution" in the sense that some npm packages are for example, where if you send any PR, it will most likely be merged and then you get write access to the repository. If you're curious how you can contribute, check out this page: https://system.data.sqlite.org/index.html/doc/trunk/www/cont...
Personally, I prefer projects that are limited in what contributions they accept to the main trunk. Code quality tends to be a lot higher in those. As long as the source is open so I can freely maintain my own patches, I have no qualms with that workflow.