The importing and exporting capabilities are really up to you. I don’t know of any 3D software that exports levels to engines by default (although they all have object exporting capabilities) and certainly the engines themselves don’t have any exporting capabilities, but every engine has importing capabilities. Technically you could just treat your entire level in your 3D software as one object, but on import this would be a nightmare for several reasons; one is that unreal by default creates collision for you, which is nice, but it does it per object so your collision won’t be even close to usable, and this would be disgusting to deal with for even a small level, let alone a giant level like you’re talking about.
The solution is to write your own importers and exporters. I use blender and unreal together. I wrote a script for blender that export levels for unreal, and a script for unreal that imports and loads levels made in blender. The ideal way to do it is to have your exporter create a custom scene description file (mine just has the obvious transformation stuff (location, rotation, scale) for the specified object) and exports each object that gets instanced at least once in blender. Then the unreal script reads the scene descriptor, imports the necessary objects, and creates each objects instance according to the transformation in the scene description file). This makes later adjustment nicer. There are ways to cheat this that are a lot less optimal but are easier (although this isn’t too much work anyway).
One upside to this is that you do almost all 3D stuff in your software, and so if you do decide to use a different engine, all you need is a new importer that can read your scene description file.
Like other people have said, this project is pretty unrealistic, so I would definitely plan on just making a city (which is already a big task). I think the idea of just doing it gradually over your life is a good mindset. Games take a long time to make, but a little bit every day goes a long way, and when it starts to take shape and look like what you envisioned, it’s a really cool feeling.