I need clarification in GST and also accounting

Yes. You are right.

To adjust the sales returns and purchase returns, those can be used..

You can also use amend records and directly reduce the revised value of invoice

Your interpretation is correct, however,as per section 34 of the CGST Act only Supplier Can issue Credit Note or Debit Note , for the reasons mentioned in the section 34 of the CGST Act, so in GSTR1 filed by the Supplier will report all Debit Note or credit note as the case may be for a perticular tax period.
In other cases except reasons mentioned in section 34 of the CGST Act, one can issue Financial Debit or credit note.
please refer section 34 of the CGST Act.