He had it set up with a "Dome" stamped. The trick is getting the settings on the HDRI IMAGE correct if I remember right.
You must choose the light type choice. Sphere, Box, Coordinate.....I think there are 3 choices.
In A:M/ chor:
One uses Global Ambiance type = Image based lighting, with an environment map (any ol' image you want to use, HDRI or otherwise) and a mapping type: latitude-longitude, light probe, mirror sphere, cube map (cross). Depends on how/what the image is that you are using has been created. I usually select lat-long and use any image that has the right coloring that I like. But I don't go for physically based lighting. light probe images, mirror sphere images are usually physically based.
Regardless of image being used for IBL, environment map (no dome):
You set the Ambiance Intensity % (optionally also can set exposure, azimuth, I never do)
and can optionally use Ambiance Occlusion (or fakeao or ssao). If you want to use A:M native Ambiance Occlusion, then yes, you must set chor/camera/render options/Ambiance Occlusion = ON. For just Image based lighting IBL (no occlusion) - it can be OFF, and Ambiance occlusion =0% in chor.
It's not that hard. Very easy.
Not necessary to make separate renders for characters and background. You can use whatever background (camera roto, or dome with image mapped to dome) you want in A:M. The image used for background can be different from image used for environment lighting (IBL). If you want to use different background (can't decide) and composite later, then yes, make 2 renders - character and background.