If you need to put together data from different sources, chances are high that they are not using the same coordinate system, the same projection. Thus, you need to reproject affected layers. For a multitude of layers done one by one this is tedious and error prone work. QGIS, however, provides batch functionality to do so:
I believe, that the layer id is the name concatenated with a hex form of a UUID (https://en.wikipedia.org/wiki/Universally_unique_identifier#Format). Therefore, I just cut the last 37 characters from the layer id to obtain the pure layer name. In some QGIS documentation there is stated a layer_name variable, but I could not find that in my version, thus the substring hassle: '/home/thiemo/Downloads/auswandern/sverige/SCID_v4.0_EPSG4326/' || substr(@INPUT, 1, length(@INPUT) - 37) || '.shp'
