You actually need to create a 3rd table called “Line Items”, which links to both your “sales” table and your “products” table.
In this Line Items table, each record would be linked to both a specific sale & a specific product. And you could also add other fields for each record, such as quantity ordered.
(You’ll have to choose a primary field too — your primary field could potentially be a formula that equals the product name combined with the sales number.)
It might be easier to do data entry from this new “Line Items” table, because it’s a little cleaner & easier for users to understand.
But this is how you would do the data entry back in your “Sales” table: Back in your “sales” table, you will be able to see all of your linked “Line Items” in a new linked field that was automatically created by Airtable. (If you don’t see this new linked field, you’ll find it under “hidden fields”). Adding new line items gets a little confusing when you’re trying to add them from the “sales” table, because when you click into this “Line Items” field to add another line item, you will actually see a list of all the other line items for all the other sales. Just ignore all of those, and simply create a new line item.