Airtable's copy and paste functions (C and V) are one of the simplest ways to duplicate your data and fields.
(To learn more about Airtable's other keyboard shortcuts, see this article.)
If you would like to append data from Excel or Google Sheets into an existing table in an Airtable base, you can simply copy and paste (C and V).
Note: Make sure that the order of the columns in the original spreadsheet matches the order of the columns in the destination Airtable base! Otherwise, your data will be placed in the wrong fields.
If you copy data from one Airtable base and paste it into another Airtable base (C and V), the pasted cells will create new fields of the same type as the original cells. (Foreign keys from a linked record field will be treated as plain text and put into a single line text field.)