The best explanation of how Ookla's Speedtest® works is probably given by Ookla itself in the YouTube video Ookla Speedtest from April 2015.
In this video, it is stated that speed test stats could be found at netindex.com. This site is nowadays redirecting to the Speedtest Global Index.
Interesting that in How does Speedtest measure my network speeds? it says (...) During the first half of the test, the client will establish extra connections to the server if it determines additional threads are required to more accurately measure the download speed (...). Speedtest apparently adapts to the speed to be measured.
Perhaps even more interesting is that the current documentation no longer shows Speedtest measuring 90% of maximum speed.
Specific measures
Ookla's Speedtest® is so popular that specific measures are needed for Speedtest® to work properly.
- Ookla uses a large Speedtest Server Network for their Speedtest® to guarantee their customers accurate and reliable measurements.
- July 2021, Amazon Web Services (AWS) created a YouTube video in which is explained how Ookla uses AWS to process 4,000 reports per second while providing near real-time insights.