VBA Comparing Single List to Many Lists & Generating A Text as Result

Thanks, where in the code can I change the location of the data in Column A, say I want this code to work if my data was on column C. I have other sheets as well so where can I specify the sheet and the column location in this code.

Also where in the code covers " I'd like to have all the information in the whole of column A (taking into account that column A may expand as it depends on the data that is fed in from another sheet - thus the selection of column A has to be able to expand automatically) "

Sorry, if I'm asking a lot of questions. Would like to understand better. Thanks a bunch !