MAC is easily spoofed, most smartphones do it already by default - although only per session

The WLAN AP collects that. They really track you (they being the AP and Google).

You may assume that they collude, or not.

It isn't, parent is making stuff up. Browsers do not offer an interface that is exposing that information.

And remote servers are outside of your local network and thus cannot see these values, either.

That's true for browsers, but Google controls both the Android OS and Google Play Services, giving them access to hardware identifiers on Android smartphones. Given the broad adoption of Android devices and the potential to correlate data, this is not a case of "making stuff up." Even if your MAC address is spoofed/randomized, the remaining data points are still sufficient to track you.

Doesn't make sense to track and correlate the mutable MAC address when you have access to the burnt-in device serial number and IMEI.

Depending on your network configuration I could imagine abuse of EDNS(0). This is used for example by NextDNS to identify which device (MAC) on your local network sent the request in order to apply specific filters and log the request. A not-so-friendly DNS could sell such information.

I use dnscrypt with a supposedly secure configuration, is that not enough to counteract this?

Ask your friendly local CIA agent, not us. We don't have access to that intel.

How do you know? Someone might!