All Networks Map Downloads / Files

They are not going to publish their maps.

Their official design team makes them and it is their property. There are already plenty of knockoff servers that use maps downloaded for free or even bought off shady sites that steal similar property and thus it would pose a financial problem as well as a security issue.

While I understand your frustration, and I'm sure they do too, it just isn't a good choice to allow. You could try looking through the old news announcements on the forums or Discord to find some renders and/or cool screenshots. If need be, you can also use F1 in-game with a small cage that doesn't obstruct your vision and try looking for an empty game, then take a screenshot while you're in the cage.

Alternatively, you can simply use a (blurred) screenshot of a part of the main lobby with some nice overlays using assets to let the viewer know what game the video about.

I think a great alternative solution/suggestion would be for the team to add some renders of various games and maps to the Notion board's Creator Assets page, where you can already find a bunch of assets to use in thumbnails. That would solve the issue of not having a decent background for thumbnails or in videos generally.

You can use mods such as ReplayMod or Flashback to make screenshots for thumbnails
Note: Mods work only in Java Edition