This video results from the 'bug' that we discovered during the lecture. I was not able to reproduce the spatial join of data that worked on my personal machine. The reason was that the projection systems of the layers differ, what did not matter on QGIS 3.14 on Linux, but resulted in a wrong result on the QGIS 3.4 on the windows machines. To prevent such issues, it is always a good idea to keep those layers that you would like to use in analysis in the same CRS. To achieve this, you probably need to reproject the vector layers. How this can be done is explained in this video.