It counts what already moved. It does not go and fetch anything.

macOS keeps a running byte count for every network interface on your Mac. Pinned asks the kernel for that list once a second, through sysctl(NET_RT_IFLIST2), and reports the difference between one reading and the next. That difference is your speed. That is the entire measurement.

Those particular counters are 64-bit. The easier interface to reach for hands back 32-bit counters, which wrap every 4.29 GB. On a fast link that can be a few minutes, and every wrap would land on screen as a nonsense spike.

Pinned opens no sockets of its own to do this. No probe, no test file, no server at the other end, nothing added to the very traffic it is trying to describe.

Which interface it reads follows the system default route, re-checked on every tick. Unplug Ethernet and the readout is on Wi-Fi. Tether to a phone and it is on the phone. There is nothing to configure, and a reading it cannot trust is reported as zero rather than guessed at.