Closing "outside faces" in extract from google maps

Hallo there!

I am quite new to Blender and came here by accident wanting to do stuff for MSFS21.

So to keep it short: i am trying to work with extracts from Google Maps (see atached file); looked several Tutorials and understand … uhem … quite the basics.

I want to have all the “outer” faces (the open tree; the open house etc.) to be closed on the z-axis like in a diorama.

First thought was that i can achieve this directly with the bool-tool but that didnt work out well. Then my next thought was i could do it my extruding on the z-axis and than scaling to 0 to have a “flat bottom”, but that also didnt work mainly (i assume) because i cant select all edges/vertices near to the “outer border”.

Another idea (and in my thought the best?) would be, do “weld” a plane to the “border” and cut away the rest - but to be honest i am not coming near to this …

So i would be very thankful for any ideas; Tutorials or even if somebody tells me that is absolutly out of (my?) possibilities …

Thanks in advance!

If someone cant or dont want to use dropbox let me know - as a new user i am not allowed to upload directly …