>On the other hand, I am surprised they correctly identify my "platform" as "Linux x86_64". Even though I used a windows user agent. How do they do that?

probably because your user agent faker only fakes the http header and not the javascript environment.

try printing out the value of navigator.userAgent or navigator.platform in the developer console.

Aha, it is in navigator.platform!

There is even more stuff in navigator, that should not be accessible to the website. For example how much ram my machine has in navigator.deviceMemory, how many cpu cores in navigator.hardwareConcurrency and so on.

I can see the use for that information, but I don’t like it being broadcast like that

The value of it is very high since it allows download pages to show you the correct info for your OS.

It seems that if you set Firefox's privacy.resistFingerprinting to true, it fills those with fake data.

    >>> navigator.deviceMemory
    undefined
    >>> navigator.hardwareConcurrency
    2

privacy.resistFingerprinting has a bunch of useful features (this, letterboxing, etc) that I wish were available under their own dedicated about:config options. I don't want to enable resistFingerprinting because I value being able to use dark mode more.

I just found out that the game Dead by Daylight seems to collect your font list if you install it. Surely this data goes to some tracking company for fingerprinting...

You can pull os multiple ways including looking at the packets and tcp headers. Also installed fonts will give away OS.

To be fair, your IP address cannot be used to track you much [1]. If you are using a VPN anyways, you can (and should) disable WebRTC (the website you linked shows you how).

[1] https://www.privacytools.io/providers/vpn/#info