Unlike Unreal Engine, Unity does not have built-in support for importing multiple heightmap terrain tiles, and have these layed out in the correct order. In Unity, you need to use third party plugins for this, or do it in a very manual and time consuming fashion. UE has out-of-the-box support for importing a tileset - a terrain split up in multiple raw heightmap files.
You should read about World Composition in the docs (link below) to learn how this works, and also which resolutions you need. Using this method you can easily create a 100 miles x 100 miles landscape.
But first you must have your terrain’s heightmap data, before you can actually import it. There are plenty of sources (google) where you can find real-world topology data.
Here’s is one source: http://gdex.cr.usgs.gov/gdex/
The Unity plugin uses Bing Maps.
You would download the image files you get from here, import them in World Machine and further refine your data, removing small spikes/peaks (ugly), and then export as tiles. World Machine can also export textures and splatmaps. Overlay textures from World Machine, are usually used only for coloring your terrain, and as a distance texture. Splatmaps will define where your more detailed textures go. This is a whole process, you must first learn to use World Machine, then learn the parts you need in UE for this, namely World Composition and Material Editor. You’ll find many tutorials on youtube which show how to setup your material.
I’ll include a link below, which describes the process World Machine to UE. This is just one workflow you could use, but knowing what you know now, I’m sure you can easily discover other.
World Composition:
World Machine to UE
Legal Issues
You should be aware that satellite imagery usually is copyrighted, you are not free to use it in a commercial game without permission/license. This includes the satellite data that the Unity plugin WorldComposer uses.