How to get POI by GPS coordinates(latitude and longitude)?

It sounds as if you might be looking for Reverse Geocoding, something you can do with Nominatim. Nominatim has a JSON API, so you can use standard web access methods to call it from Java or C#.

If you plan to publish software that makes frequent use of Nominatim, you should probably set up your own Nominatim server instead of relying on the donation-funded server operated by OSM itself - see the usage policy.

Do you want to get the POI(s) or "know which place is" the coordinates describe?

For retrieving all POIs of this area: Use Overpass API. Let's say your coordinates are lat 51.56100 and lon 10.83576. The following Overpass API query will return all OSM elements within 25 of these coordinates:

[out:json][timeout:25];
(
  node(around:25.0,51.56100,10.83576);
  way(around:25.0,51.56100,10.83576);
  relation(around:25.0,51.56100,10.83576);
);
out body;
>;
out skel qt;

You can view the result at overpass turbo. Note that you can retrieve these results programmatically (example) because overpass turbo is just a nice web frontend to Overpass API.

For knowing which place/address the coordinates describe: Use the reverse-geocoding of Nominatim or one of the other OSM-based search engines. Example call for Nominatim for the same coordinates as above: https://nominatim.openstreetmap.org/reverse?format=xml&lat=51.56100&lon=10.83576&addressdetails=1