Inspection of gene expression in scRNA-seq data
In an ideal world, ribosomal RNA (as seen in your top hit) should be excluded from samples prior to sequencing. Where this is not possible (i.e. in the data that have been presented to you), it would be a good idea to exclude ribosomal genes (and any other common contaminants) prior to doing further analysis (including normalisation). I believe this is the point the course instructors are trying to get across with that message box.

Eyeballing the results, you should also exclude any ERCC genes as well, as they are synthetic constructs and not from the target samples (although record their counts, as it may be informative for QC/normalisation purposes); they'll have odd distributions because of this.

ERCC sequences are typically added based on the concentration of DNA in a sample, rather than the number of cells. However with single-cell data (i.e. with known cell counts), it'd be possible to do a proper ERCC-based normalisation, assuming an equal amount were added for each cell. I suspect that knowing a bit more about the ERCC addition protocol would help in understanding why their distribution is so different.