You can see the modeler selecting an edge loop (or face?) then selecting a second bit and doing some menu trickery and like magic the two snap together! This is different from ‘merge at’ as that only works per vertex, this is taking in to account the original position of one of the two selected edge loops/faces and snapping the verticies of one of the two selected bits on to the second. I would love to learn how to do this! I’ve looked about quite a bit but only found some really old conversations about the possible implementation of a bridge tool back in '08.