I'm not a pro, but based on what I observed, the undermount sink sits on a grooved hole in the concrete. Then you just top it up with granite or quartz or solid surface, with an opening for the sink
Top mount: easy to replace if your sink stains or rusts.

Undermount sink: water is easily swept into the sink as your top will be one smooth surface up to the sink opening. Looks much better too IMHO. But must make sure proper sealant is applied, otherwise will be troublesome if it leaks from the undermounted edges

Please correct me if I'm wrong