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Flathub Grows Past One Million Active Users
(docs.flathub.org)
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Distributions include the Linux kernel and supporting system software and libraries, many of which are provided by the GNU Project. Many Linux distributions use the word "Linux" in their name, but the Free Software Foundation uses the name GNU/Linux to emphasize the importance of GNU software, causing some controversy.
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I don't think there's any business entity artificially forcing the users to use it (like Firefox on Ubuntu 😉) if that's you're asking.
Otherwise, the only case where the user is "forced" to use flatpak would be when the software they're looking for is not available under their distro's repo, which happens a lot especially in point release distros.