How is Google Meet able to show CPU usage?

Most probably, they are using Chrome specific API since this will not work on other browsers. The closest thing that I've found is https://developer.chrome.com/extensions/system_cpu#method-getInfo

Below is a screenshot from Firefox stating that this will work only on Chrome.

I was curious about this as well when I saw the CPU Usage graph on the Troubleshooting page on the Google Meet's page. So googled a lot and I couldn't find any proper answer as to how it is being achieved apart from this link: https://developer.chrome.com/extensions/system_cpu#method-getInfo.

So, I went through the minified source code of the Google Meet's site and it turns out Google Meet is using this but not on their webpage directly, because it is for chrome extensions only, but through the Google Hangouts Extension which is present in Chrome, Brave and the new Microsoft Edge (Chromium-based).

The chrome extension is calling the chrome.system.cpu API to get the information about the processor, core count, temperatures, etc., and these details are being passed to the Google Meets page via window.postMessage API and Message Passing between Chrome Extension and a Webpage.

This is a neat little way to achieve this if you are the creator of the browser :P.