Yeah, I don't mean to say Visa can run global payment processing on a single postgres install; I'm sure they do a ton of stuff with each transaction (e.g. for fraud detection). But for system design, it gives an order of magnitude for how many human actions a global system might need to deal with, which you can use to estimate how much a wildly successful system might need to handle based on what processing is needed for each human action.
For similar scale comparisons, reddit gets ~200 comments/second peak. Wikimedia gets ~20 edits/second and 1-200k pageviews/second (their grafana is public, but I won't link it since it's probably rude to drive traffic to it).
interesting re reddit, that's really tiny! but again, I'm even more curious about how many underlying TPS this turns into, net of rules firing, notifications and of course bots that read and analyze this comment, etc. Still, this isn't a scaling issue because all of this stuff can be done async on read replicas, which means approx unlimited scale in a single-database-under-management (e.g. here's this particular comment ID, wait for it)
(still, modern postgresql can easily scale to 10,000s (plural) of TPS on a single big server, especially if you setup read replicas for reporting)