As a shut down online mobile game, a lot of the content once available in Minecraft Earth isn't really accessible anymore because it wasn't part of the game client but downloaded separately as-needed off now-dead web servers. Buildplates and Adventures were downloadable sections of terrain that could be used for building and adventuring (shocking, I know) and there were a lot of them. Many of these that people downloaded before the game shut down have been archived on various websites (like this one https://craftersguide.stents.dev/) but I don't think all of the ones known to have been released are accounted for since many of them were made unobtainable at some point before the game's shutdown.
More information:
https://minecraft.wiki/w/Earth:Buildplate
https://minecraft.wiki/w/Earth:Adventure