but I just don’t know how to see it in a graphical interface and extract the snapped GPS coordinates?
You can have a look into this JS code: GitHub - graphhopper/directions-api-js-client: JavaScript client for the GraphHopper Directions API and here is the client directions-api-js-client/src/GraphHopperMapMatching.js at master · graphhopper/directions-api-js-client · GitHub
The important part is probably to convert the polyline (in your example "points":"wud{Iagr{@PVBV") - you can do this via the decode function in the JS code or less efficient would be to specify &points_encoded=false when POSTing the data.