It's complex really, but one big difference is the license used.

With a BSD license you can keep your code closed, that's why iOS is closed source for example.

Linux uses GPL, which means the source code should be available, which you can get from AOSP: https://source.android.com Android derivatives like GrapheneOS use AOSP as their base for example.

There's no prevention of contributing to it. What I mentioned was the specific laws in the US about export of cryptography. You can't export certain types of software from the US to, say, Iran, or Cuba, etc. But those laws don't apply if the software originates from say, Canada, which is the case of OpenBSD.