You can take a look at my flight demonstrations to see what kinds of things you can do with three.js. Over the years, I have created several open source modules to help programmers create their own flight simulation.
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You will notice that I have not implemented a full 3D terrain. Instead, I am adding 3D models of islands and mountains on top of the water or terrain. The flat ground texture I am using has a resolution of 1/2 foot per pixel, which is very high and helps with estimating your height above the ground (along with the shadow).
There are many good examples of full 3D terrain, including an official three.js example and many examples on this forum and on YouTube. One YouTube example used a nicely detailed terrain (of limited size) created entirely in Blender. So you will have many options to choose from.