IIRC, it uses the native Adobe FBX C++ SDK and builds out to a GLB that only contains the mesh, PlayCanvas material assets, the texturesimages, animation GLBs and template that hooks it all up in a entity hierarchy graph.
It’s closed source unfortunately and also not runtime available.
I don’t know of a FBX to GLB runtime convertor
If you would like to discuss it with the PlayCanvas team, best to do so via support@playcanvas.com