Who is familiar with payment terminals?

Question

Payment terminals

Today, while walking around the store, I noticed a couple of details, in some stores there are whole computers with readers, check machines, etc., while in others there is only 1 payment terminal the size of a smartphone that allows you to put in a card, I didn’t understand this a bit:
1 ) How is a barcode reader integrated in complete systems? Are there open source projects?
2) How are payments made in small shops? After all, this terminal only allows you to insert a card and write a pin code, but how does the terminal know how much to "withdraw"?

Answer 1

In full-fledged systems, it is easy to use special barcode scanners.
Many of them are "inserted into the keyboard", that is, when reading a barcode, they emulate pressing numbers on the keyboard by entering the product code in the required field.
In small stores, they make two checks at the same time - they punch a check at the checkout and punch the payment through a small terminal, entering the amount manually, the main thing here is that the amount matches