Anyone can have an Apple Account whether or not they own an Apple Device.
In this case, too, you can create Invites on icloud.com on non-Apple devices. Including the webpage seems nicely responsive and can probably make them in an Android Chrome tab if you wanted.
The only remaining obstacle is that it isn't a free feature of an Apple Account, but requires an iCloud+ subscription. But that's useful for Apple Music and Apple TV+ and other products, too, many of which work just fine on non-Apple devices as well.
Also, GNU/Linux phones exist (Librem 5 is my daily driver). However without Apple's budgets, you can't create the same smooth experience. You just can't compete with the duopoly.
Yes, it does: https://puri.sm/posts/breaking-ground/. Purism tried to created their own smartphone not relying on Apple and Google and it was almost impossible to find the necessary chips. Nobody wanted to share the schematics or open the drivers. People are just locked-in into the duopoly. It's impossible to use popular apps without it, like Whatsapp or even Signal (!).