The only reason (as far as I remember) that players are stuck with the demo is if they haven't signed in with their Mojang/Minecraft account. Try logging in on the launcher or on minecraft.net and it should give you the game properly. Twitch uses the regular Minecraft launcher Mojang gives players so it works the same only you use the Twitch Launcher to go about mods and other games.
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'Niche Community Content Finder' for fun and support of creators. I look for mods that interest me or need a bit of acknowledgement or bug fix testing/reporting. I also make modpacks/maps
Creator/Maintainer of Fabric, Rift & Tesla Mod Discussion Threads/Niche Mod Youtuber.