Let's skip the whole bit of me still living there and come to the post's subject
The culture of paying for stuff there is complicated. While at most places like general stores you can pay with both cash and cards/QR, and that goes for like 15 years, some places prefer one mode exclusively, e.g. privately-owned buses do only cash or newer shops not having a traditional cash register at all. You learn that as you go, and I generally avoid the second ones.
The difference doesn't stop here though, and depending on infrastructure leverage, stores either use their own hardware to accept payments, or they use one from Sberbank, the main bank in Russia, for a fee.
Southern mega store chain Magnet uses their own scan panels you put your card onto and what they have on you, especially if you use a loyalty card to link all your purchases to the same account, is with them and them alone.
But smaller chains and single stores use Sber infrastructure and their card readers. While these accept any russian card, there always was a fear that they still process and look into every payment even if you aren't their client.
Moreso, the recent models of their card scanners are equipped with a camera, and they can and do scan your face. I was oblivious to it for some time, but recently I got a very urgent reminder.
I bought a jug of beer at a small chain of breweries and when it came to paying for it, their Sberbank scanner got into "pay by smile" mode. I didn't notice it at the time, and even put a card of another bank to the scanner, but when I got home I realised it took money from my Sber account, from the card I never carry with me.
So, unknown to me, they got my face biometrics and billed me based on that. With how many places have these Sber's readers, I got tracked by face in almost every place providing products and services, with enough precision bank fiends trust it. That's cops' paradise.
Keeping in mind I'm still yanking myself out of serving for obvious reasons, I'm pretty terrified that if they'd want me in, they'd have a realtime report of where and when I bought stuff, they can take me on exit. There were leads, like triangulating my simcard usage or payment history, but they weren't as firm as a picture of my face at this place at this time. That's the whole another level of surveiliance you'd hardly dodge as long as you need food or water.
I don't know if it's the case in other countries, but I want to to warn you that it can happen, and it's even worse than Flock.