why does the minecraft launcher say I do not own minecraft?

Hi, I hope you're doing great out there despite the issue that you're experiencing. I am an Independent advisor and I am here to help you in regard to your concern.

Kindly check on your profile if the game is showing from there. Make sure that you're also signed in under the correct Microsoft account.

https://www.minecraft.net/en-us/msaprofile

For me it opens up the Launcher recognizing my minecraft name but for the Java version (which I previously owned under Majong) it says I can PLAY DEMO. For the Minecraft for Windows it says I can PLAY TRIAL. But I already own Minecraft.

Not sure why your Java username is showing as already taken, but since the Minecraft website shows that you own Java, this is probably just an issue with the Launcher syncing data.

Would you please try to reinstall Minecraft Java and the Minecraft Launcher too?

Same here, I own Minecraft for Windows 10 but nothing recognizes it. Minecraft.net claims I do not own it, the Xbox app says I own no games, and even though I can sign into the launcher it says "Failed to create profile" whenever I press "Play"

After uninstalling and re-installing everything, as well as signing out of every account, the only difference is that I am now also unable to install the Minecraft launcher. I installed a fan-made launcher and it also claimed I did not own the game.

If there is a way to fix this I would greatly appreciate it.

If you want to be sure, keep with the no. I'm almost pretty sure this is not possible for Minecraft since Minecraft does not uses the Xbox app to sign in, but its own launcher.