Hey guys. Just thought of adding this note.
First off - Ambient Occlusion is something that software does to fake the soft shadowing when meshes interact with one another. The Engine can do this shadowing by itself without having to do AO pass - but, you see, it is very resource intensive to do this soft shadowing with just lighting, you end up dropping performance if you do this. So that’s why the concept of doing an AO pass exists. Screen space and Raytracing renderer uses AO to do this shadowing effect (as a bonus, artists get to art direct this pass too - like controlling falloff radius, intensity and colors. You see the concept here? You are not actually modifying your lights for these effects. This is all just a pass generated by engine being composited on top).
If you take a pathtracer renderer like Arnold or even UE’s pathtracer, it does this self shadowing without having to generate an AO pass. The downside to this is that you lose artistic control of AO because the AO pass does not exist in the first place (that’s what I think at least). Lumen however uses a technique called 'Virtual Shadow mapping" to achieve this look. You can visualise using the view modes on your viewport too.
But for more stylised work, I think Epic should add an ability to give AO pass and be able to composite on top of final image in Engine itself. For now, the option is to take the AO pass from SS or raytracing GI and composite in some editing software like AE, photoshop or Nuke.
EDIT: User “cschrubini” has linked a video in the below comments showing how to enable SSAO with Lumen.