Having trouble creating macro to compare monthly reports and calculate differences

See attached file for example.

Each month we have to run a report and I thought it would be a good idea to be able to analyze the differences in the reports on a monthly basis.

Ex. first sheet we downloaded in Nov. 2021
- have several different products, with values corresponding to each month

second sheet is ex. of what was downloaded in Dec. 2021
- have some similar products, some different, with new values for each month

Ideally what I want to be able to calculate is sheet number 3 (the comparison) and be able to see the biggest differences (like in tab 4). Too hard to do manually as there are normally thousands of lines, and the products are not always the same.

No idea where to start with a macro so I'd see if anyone here had any advice.

Thanks in advance

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