I am quite new to GIS software. I use QGIS. I have a question that might be trivial. I would like to derive the coordinates (Y and X) for an area. Just to make an example: I have two regions A and B. From the shapefile I know which are their coordinates. However, I would need the coordinates for the unique region A+B. Could you please give me some hints on how to do that?
From the precisions you provided in the comments, I understand that what you want is actually a polygon merging followed by some kind of vertices coordinates export. This is actually quite simple:
Merging: Select your layer then click on the pen icon to toggle edition mode. Select your two polygons, then go to Edit > Merge Selected Features. This will replace your two polygons with their geometric union (so if you want to keep your old polygons, make a copy of the layer first). This will work well if there are no gaps or overlap between your polygons.
Coordinates export: there are (at least) 2 options. You can right click on the layer, then "Save As..."; in the "Format" field, choose CSV. In "Layer options", "Geometry" field, choose "AS_WKT". Alternatively, you can install and use the MMQGIS plugin (see this answer for reference).