What stays in the tab
Microphone samples, camera frames, keystrokes, pointer movement, and controller state are read locally so the page can draw a result. Leaving the page stops the tracks and drops the recording.
Privacy
Audio, video, and keystrokes from the device tests are not uploaded.
What can leave the tab
The speed test sends random bytes, then the server discards them. A meeting report can save four scores and nothing else, for seven days. Those requests do not include a recording.
Visit measurement
Google Analytics records that a page was opened. It sets a cookie in this browser. Device-test audio, video, and input stay on this device and are not part of that measurement.