Merging polygons of different layers with QGIS

Question

On a map, I've created several polygons to mark some areas. Each polygon (shapefile) is on its own layer.

How can I merge the polygons, so that one layer consists of multiple polygons?

I can't select most of them (the "select function" is not clickable) and even the ones that I can select and then try to merge, it says that at least two objects are required in order to merge (even though I have marked two). Why is that?

Answer 1

You need to use a tool for merging layers, not for merging features on the same layer.

Try the "Merge vector layers" tool ("Vektorlayer zusammenführen" in German) from the Processing Toolbox ("Verarbeitungswerkzeuge").