I want to draw a city (Avignon, France) with JavaFX and OpenStreetMap (via Overpass API). What is the QL request that I have to send to Overpass API to get this information?
Overpass API is not really the right tool for querying large data like all buildings and roads. It is rather suited for making specialized queries like all postboxes in an area.
You can do a one time download via https://www.openstreetmap.org/export#map=14/43.9402/4.8096 -> then click on the "Overpass API" link. This area is small enough to be downloaded via Overpass API, the resulting XML will be around 100MB.