Back in Lucy OpenDev, it was possible to build districts, as long as they are adjacent to any kind of already existing districts, including remote ones like harbours, forts, mines... As of this build, it is now allowed to build next to administrative centers or completely connected districts (to city center, so that the entire districts would have themselves behind the wall). I am not sure if it were testing a new approach or a permanent change, but it was also interesting experience.

But it is impossible to place anything on the yellow hexagonal tile.

I think at the minimum, if the districts are completely connect to city center or administrative center, they should provide valid spots for placing districts. Otherwise, expanding a city beyond that tile would forcibly cost 1 resource, since placing other valid districts would prevent extraction of the resource.