You are using Mastercard's payment network whenever you pay for something. They see and record all your transactions. Mastercard is the entity informing your bank to pay the merchant's bank.

Yes, but how is Mastercard encouraging me to spend more? In my day to day, it matters little which payment network I use since that’s abstracted away by the bank’s interface. What opportunity does Mastercard have to target me?

Presumably they could tell stores that you have your card on file with? (Only speculating; I have no idea if they or any other card network actually does that.)

Offering lower interests rates as a promo, higher limits, low interest loans, bonus points from certain brands/categories, offering new types of cards, etc etc

Read OP’s comment again. Mastercard is not a card issuer, they can offer none of the things you mentioned.

They sell your transaction data to Google. When merchants provide Level 3 information (line items), Google can now know exactly what you're actually spending your money on.

Then, Google can show you super-relevant ads, that might encourage you to spend even more.

My worry is that that merchants offer higher prices based on their assessment of my private data.

We've had the situation before when booking holidays that my wife sees higher prices for the same hotel on her laptop while sitting right next to me. Once she cleared cookies the price went down to match what I've been offered on a clean computer.

That was a few years ago so I would imagine IDing potential customers is done via browser finger printing now rather than cookies and so harder to protect against.

Really, who wants their bank or payment network to collude in higher prices?