Who has experience in implementing an open seamless world?

Question

C++ / C#

Hi friends.
Interested in real experience, some kind of infa.
I found a lot of information on the Internet, but there is still no adequate solution.
Mobile game. An open world, you need to load parts / regions, because the city is very large and if you load it immediately, its FPS drops to zero.
There is a ground (ground) and buildings on it.
Has anyone tried splitting regions into separate scenes and uploading them? I tried a couple of regions, but there are some drawbacks - such as download prolags.
Who implemented it? Let's say if the games are big like Skyrim.
In addition to the issue of loading the regions of the earth, are buildings also of interest? For example, you need to draw buildings with Pts. long distance, are there boats here? imposters?