I have two attribute tables, 1 from a shapefile layer (left) and 1 from a csv layer (right). I would like to find values in the first column ("KEY" column in shapefile layer) that appears in the second column ("KEY_ID" column in the csv table layer). I will likely have them selected and exported as a separate csv file, containing only those IDs that appear in both columns.
You can achieve this by joining the shapefile with the CSV file based on the common fields between them (KEY field and KEY_ID).
To do this, add both shapefile and CSV files as layers to the table of Contents using Add vector files, then
right-click the shapefile -> Properties -> Join -> Add join (green plus at bottom left) -> Select the CSV file as Join Layer, KEY_ID as Join Field, and KEY field as Target field from the shapefile.
Now the field that are joined are the identical KEYs that you can use for further analysis.