They accurately measure your connection to this specific server at this moment - real data is transferred and precisely timed. But "speed" always depends on several things at once, such as your provider, your WiFi, other devices on your network, and the distance to and load on the server, so treat the numbers as a solid real-world snapshot rather than one fixed, universal number for your line.

Most home internet connections, especially cable and DSL, are designed asymmetrically on purpose: providers allocate far more bandwidth to downloading, such as streaming, browsing, and downloads, than to uploading, since that is what most people use most of the time. Fiber connections are the main exception and often offer equal, "symmetric", upload and download speeds.

It's a live network measurement, not a fixed number - WiFi interference, other devices using your connection at the same time, your provider's network load, and even how busy this server currently is can all shift the result between runs. Running the test a few times and comparing gives a more representative picture than any single result.

Technically yes, it will run and produce a result - but that result measures your connection to the proxy plus the proxy's own connection onward to this server, not your raw connection speed. If you want to measure your actual internet connection, disable any VPN or proxy first; if you specifically want to know how fast your proxy connection is, running it with the proxy active is exactly the right way to find out.