I want to create an open world game and I want to know the best way to build the terrain. When modeling this world out is it best to make one massive mesh, tons of chunks that fit together, or to use a height map and build everything out of tons of meshes - such as using a set of boulders to make a mountain with a cliff?
That’s a very broad question, and it depends on so many things. And at the end of the day it really is just how you wish to do it and performance you want. You want to be careful with landscape component count and size. (documentation gives recommendations here)
I would recommend a couple of things.
Check out the “Kite Demo” from the launcher. (epics specifically built open world demo, for this purpose)
Download the ARK mod kit from the launcher, and take a look at how it is done there, you don’t need the game to run this. (actually this might just be the best resource you have, and it’s free!)
Learn world machine, or something similar.
Learn about world composition, and how to import a tiled landscape (there is some wiki articles on this)
Most games would be laid out with 1 large base landscape, split into tiles for streaming. Which would be generated in WM or something.
Then with meshes used for extra details. Cliffs/Mountains, etc. (extra detailed areas etc)
Also, bonus learning:
Watch some GDC talks about how studio’s do it. There are a few talks by people such as Ubisoft that go into details about how they do it. They did a talk i think this year and how they approached ghost recon wildlands, which to my memory is a 16x16k landscape. (if you are doing this by yourself, i’d recommend you don’t make something that large, lol 
Now chances are you won’t do it anything like this. But those talks are good to get you thinking and give you ideas.