In-browser DRM usually uses a library called Widevine, which is a closed-source library created by Google that's usually only used on Windows or MacOS.

On Linux, you can use Google Chrome to get Widevine working. You can also extract the library from Google Chrome to use it with Chromium (e.g. see https://github.com/proprietary/chromium-widevine). The version of Chromium shipped with Linux distros doesn't include it since you need a license and permission from Google to distribute it. Lots of Linux users would also (understandably) really not want to run a DRM binary on their system. It's intentionally obfuscated to try and prevent people from breaking it.

I don't know what other Linux browsers do - I haven't used Linux desktop for a while (going to switch back soon though). On other OSes, browsers like Firefox and Brave prompt you the first time you try to watch DRM'd content, asking if you'd like to download the plugin. I assume they license it from Google.

Also as far as I know, Widevine doesn't allow the same security/compliance levels on Linux as it does on Windows and MacOS, as the OS is less locked down. This could mean that a 4K video streaming service works fine on Windows but won't allow you to stream in 4K on Linux. Isn't DRM great???

Sure, but the thing is: only a single person needs to break it temporarily in some way and this person can then leak the DRM free copy for everyone to consume.

That's why DRM is such bullshit. It only ever punishes legitimate users. All others are unaffected.