Everybody’s process is different.
I like to start with World Machine Pro. This will generally give me a solid starting point. From there, I will sculpt in custom landscape features like caves or anything that looks to have been touched by man.
Adding in vegetation and grass, I’ll “paint” in a variety of models. This could be done procedurally, but you really want to control this stuff if you’ve got gameplay & style concerns. Just keep in mind that you’re going to have to work with LODs and other such shortcuts if you want to manage frame rates and memory.
Regardless of whether you’re using Unreal, Unity, or any other engine, never forget to profile. You’d be surprised how quickly a forest can bring your PC to its knees.
Again, everybody’s process is different. Some people have a huge assortment of terrain brushes and might work out the entire landscape in ZB, forgoing a terrain generator like World Machine, Terragen, or Vue.
(FWIW, World Machine Pro has a somewhat old looking GUI, but it’s cheap, fast, and powerful. You have to add your own trees and plants, but that’s not a huge deal depending on how you work.)
Unreal handles tessellation and displacements. Which technique you use is up to you and your environment. For large open world environments, scalability is your friend. For more narrowly focused envs or demos, hand optimization might suit you better. TBH, it all depends on your world map and presentation style. You’re going to have to test and profile.