If it's not too much to ask, I'd very much a "bullet point summary," as it looks quite interestting.
Geometry nodes is a procedural 3d modeling/assembling, I might take some time to understand how to use it but once it's done it has huge potential for buildings, or cities creation.
The guy is a little to chatty, so this tut looks very long, but you can keep skipping those parts and look only how he builds his nodes. Around min 40-50min of this video, he is completely lost and trying to find how he went to certain point, how he built that nodes tree. After that he explains how to render it, this part you can ignore and this means this tutorial is much shorter than it seams at first.
He is using his own asset or 3d part exported to fdx format. It look like this
https://gumroad.com/l/QAYyF.
In regards to Kyriakos's cities creation, when you set once a plane shapes for all cities sizes and build your first city set, made of modular parts as per attached link, later if you keep you asset tree in the same order where each file will have the same name, then for the next city set you'll have to only modify slightly a mesh of windows, doors, etc... certain parts which holds characteristic features for particular set, let's Asian looking and swap over you existing asset, what finally when you start a Blender again will give you a different looking city.
This method give you standardized looking cites but with only small effort you could easily produce not one city set a month or 3 moths but several a day.
By the way, the guy on this tut shown only ground floor creation, there will be another tut when he will create the rest.