Bilu What they're running is not GrapheneOS, in that case. You could technically port GrapheneOS over to other devices, but it's not something that can just be done by anyone who uses the OS. You'd have to be a developer to actually maintain it and build new versions and everything else.
And with that said, even if you did manage to do that, which is very hard, your device would still not really be running GrapheneOS, as for you to be able to use all of GrapheneOS's features, you need hardware that supports them, which is why only Pixel devices are supported at this time.