Lets say I buy something at some (physical) store and pay using a credit card on one of these electronic terminals. What information do the owners of this store get about me (or my credit card) from this transaction?
Can they find out whether multiple purchases were paid using the same credit card?
From the card itself, the Merchant gets the track data, which includes card number, expiration date, and cardholder name.
If the Merchant requires zip code verification, they'll get your zip code, obviously.
(Card-Not-Present Merchants often get address data for billing/shipping purposes, but you asked about physical stores... and they get that from the Customer, not the card itself.)
The Merchant can track purchases made with that card within their store(s), but not those made at other, unconnected stores. Be aware that sometimes multiple stores (e.g. HomeGoods, TJ Maxx) are actually the same "Merchant" (TJX Companies).
The Processor, on the other hand, can correlate a single card's activity across multiple Merchants. They don't generally have transaction details ("what you bought") but they do have amounts, categories, Merchants, times, all of which may be provided to the Card Brands (Visa, Mastercard, ...) upon request, or law enforcement upon a subpoena.
Each processor will have a different view. If Processor A handles Merchants A, B, and C, and Processor B handles merchants D, E, and F, then the Processors will have completely disjoint sets of data to work with. In general most Merchants use a single Processor; some load-balance across multiple Processors for redundancy and availability, but most transactions will only be seen by one Processor.
Processors do a lot of data analysis to provide value-add, but not to the extent of providing individual cardholder details across Merchants. Most such data analysis is done on large, anonymous buckets, but others, like householding, require identifying factors be used in the analysis.
Processors, Card Brands, and Banks can also make loose inferences about what you're buying based on the Merchant Category Code (MCC). These aren't very exact - those salted peanuts from the Exxon station might get classified as "Gas" - but they provide some guidance. These are the codes that Corporate-issued credit cards will use to block non-work transactions.
Finally, cards themselves are informative. Merchants can tell the difference between a prepaid card and a Black Card, and they can treat the cardholder differently in accordance with their status, for example extending discounts to higher-value-card holders. This is true not only in a physical store, where the Merchant sees your card; Processors can provide this sort of metadata to Card-Not-Present Merchants as well.
(The ability to determine the type of card is not unique to Processors; it's based on the BIN (the first 6 digits of the card) and you can look it up with freely available tools like binlist.net. However, since the list changes over time, and since it's only a portion of guidance, this is a service most usefully provided by a Processor. For example, anyone can tell if a card is a Black Card - but as a Merchant you might treat a Black Card with a high chargeback rate differently than the rest. Only the Processor can integrate that guidance.)
At the very least, they can get the card number. Most receipts will even have the last few digits of the card number printed on them, but the system will have had the full number at some point, and may well hold a tokenised version of the card number which is allowed under PCI (think of it being a random value which can be linked back to the card number by the tokenising service). Since the same card probably gives the same token each time (technically this is optional, but since it gives more information than the alternative, it's the more common in practice), they can go "this card also bought X, Y and Z on these dates".
They can't usually cross reference that data with other stores though - the token associated with a given card from store A is completely unrelated to that associated with a given card from store B, in a sensibly designed system. I don't know whether any tokenisation providers pool data from multiple clients, but that could be a potential nightmare under GDPR, so I'd assume not, at least in Europe.
The issuing bank can also see purchases being made, obviously, but usually in a per transaction basis, rather than individual items. That doesn't mean they can't make educated guesses about the purchases (e.g. if you make a payment to a business called "99p Donuts" for 99p, it's a pretty safe guess that you bought a donut...),
Just to add to the previous answers: Apart from the obvious data that is entered during checkout, you can also get the issuing bank name from the first 6 numbers of the credit card number. With the bank name you can sometimes guess the country where the card was issued when you get a result like "Bank of China", "ANZ" or "Národná banka Slovenska".
I used this method (along with others) in an online store when calculating a risk factor before an order was shipped, to eliminate or flag orders that are placed with stolen credit card numbers. If user IP, delivery address and bank card are all from different countries then your order would go on hold and flagged for manual review.
