Linux is commonly used in the communications systems, like on invidual radio “stations”. Propably used everywhere where high confidentiality and security is required.

Real shame that GrapheneOS only supports google pixels since they are kinda expensive.

They never happened. Winners write history.

Yes, though there are some prebuilt binaries for large packages. I use gentoo on a desktop and updates don’t take too long, minutes. Big updates that cause lot of packages to rebuild can take hours.