How to make a game world?

I have always wanted to make my own version of The Simpsons: Hit and Run since I discovered this site in 2019. I have one question, how do I make a stable landscape of Springfield altogether? i have the tools and resources necessary, and if I need more, the tools are on here. I have made attempts again and again, and every time i do, the map parts are all over the place, and all jagged. How can i make them connect to each other?
Additional comments and question are also available.

Making a custom map would require you to know a bunch of things, mainly about P3D files, how they work & how to use Blender.
You can find more information on this page.

How can i make them connect to each other?

3D editing.

You should probably check out @Colou 's Fully Connected Map & how it was made.

I know it has been a while, I have gotten every map piece together. The fully connected de compiled map had missing parts inside, so I took the parts from the original game and imported the parts in blender 2.83, which is the software I am using. My question is how do I put the mesh on the buildings and objects?

hey, I want to say thanks, it did help. With that, I now have some roads and firetruck mesh on. Will continue to make it look good.