So? What if the VPN doesn’t know who you are? What if it’s your own VPN but others don’t know that?

The entire point of choosing a good VPN is to get the visible data off of your immediate ISP, since they know who you are, and into a more nebulous net, where others do not.

There are Tor exit nodes which monitor traffic see:

https://boingboing.net/2016/07/01/researchers-find-over-100-...

If the traffic is https, in theory they can't link any of that traffic to individuals.

If a party were to, by chance, to monitor both your entry and exit nodes couldn't they match the traffic by time & packet size, et al? Then use known techniques to match pages accessed (it's something like 85% accuracy IIRC).

That would match to individual IP, or potentially an individual if you're logged in.

This is a great way of thinking about a VPN, and how I describe it myself. If you have limited options on your choice of ISP, then a VPN allows you to expand your choices (excluding speed, latency, usage caps).