I don’t know about all of these things, but Signal’s source code is open source and freely available on GitHub. This includes its Android, iOS and desktop apps, as well as its server code and underlying encryption library:
https://github.com/signalapp
It also has regular security audits from third parties:
https://community.signalusers.org/t/overview-of-third-party-security-audits/13243
The last one was done on February 18 and found no vulnerabilities in Signal’s encryption:
https://soatok.blog/2025/02/18/reviewing-the-cryptography-used-by-signal/
I hope this information is useful!