M.int

You are not arguing in good faith.
I have linked multiple times to the docs and to the GitHub repository of flatpak.
Now how about you link to something useful in the docs that proves your point or maybe just a random article as source to your misinformation.

So you linked to apt.
I guess good for anyone who finds this interesting…
But more on topic here is is a link to answer from 2020 from an flatpak maintainer:

If a user installs or updates a specific app-id the code verifies that:

• The new app is gpg signed by a trusted key
• Checksum verifying that all files are untampered with
• The new app has that app id
• The new app has a later timestamp on update

You have not provided a single link.

I’m am no expert on flatpak and just did some basic searching.
From reading the command reference it seems GPG-Verification is enabled for each remote and can’t be disabled/enabled for each install. I can just find some issues where gpg verification fails

Error: GPG verification enabled, but no signatures found (use gpg-verify=false in remote config to disable)
error: Failed to install bundle fr.handbrake.ghb: GPG verification enabled, but no signatures found (use gpg-verify=false in remote config to disable)

Documentation seems to be more user oriented and not developer oriented maybe someone more knowledgeble can go in the source code and tell us how it actually works.

This seems to be blatant misinformation.
The default seems to require a gpg signature. It can be disabled for a remote with --no-gpg-verify, but the default for installing and building definitely requires a signature.
You keep talking about the docs, so please show me where is says that in the Flatpak Documentation.

flatpak build-sign, is what I can find in the documentation.