I understand that .pbf file contains the OSM map data.
What is the content inside andorra.osm-gh folder?
Also, if I have to run program for a different region, say London, then how do I generate these inputs?
The gh folder contains the nodes and edges and shortcuts that graphhopper uses internally for solving routing problems. These files tend to be expensive to generate. Every map has to have its own gh folder. You don’t have to specify the gh folder, its usually the osm file plus a suffix. Edit the config.yml for your needs and start the server with ./graphhopper web andorra.osm.pbf, given that the osm file is in the same directory.