MKV is a container format for bundling together streams of audio, video, and text. It does not provide the actual video compression, which is still typically h.264 or h.265.

The other answer you got hits it on the head already.

Here's an additional piece of info you might like: The webm container format is actually a specific subset of Matroska. The big players in the web recognised that it was a very useful open container format and adopted it for the web. They took a subset to make implementation in browsers easier and more uniform.

Believe its similar techniques to h.265 for AV1. That's still going on....not sure if it will go anywhere.

Shit I hadn't heard of that. That makes me sad. I know from reading developer blogs at the time that they tried to use less established techniques in some places to try and avoid similarities. Let's hope it proves to be enough in the lawsuit. That would be horrible if some dusty encumbant can crush the new codec at a critical time of its global adotion.