Excel Automation using LINQ - Need help to make a code

GST Portal (04 to 08).xlsx (1.7 MB)
GST Purchase Register Report vr4 (04 TO 08).xlsx (791.3 KB)

scenario1: If GSTIN in gstr2a sheet of portal file and Bill to GSTIN of sheet1 in Portal register file matches then (IGST-Total IGST Tax Amount=a),(CGST-Total CGST Tax Amount=b), (SGST-Total SGST Tax Amount=c) and write the values of a,b,c in new columns of both the sheets
scenario 2: If GSTIN and INVOICE NO in gstr2a sheet of portal file and Bill to GSTIN and Party Ref. No of sheet1 in portal register file matches then (CGST+Total CGST Tax Amount=d) and write d value in separate column in both sheets
scenerio 3: in gstr2a and sheet1 ,find how many times particular GSTIN and Bill to GSTIN is repeating i.e,count of the element