Websites can use the User-Agent HTTP header and various techniques to try to guess your OS. I don’t know what’s your User-Agent? In the default ˋdebian-12-xfceˋ template and using Firefox, I don’t think the User-Agent is enough to tell that I’m using Qubes OS. I think there are other ways to try to guess the OS, but I don’t know if it’s possible to tell I’m using Qubes OS instead of another linux distro.
If you’re logging to Google using Chrome, maybe your browser could be checking this and telling Google? The following topic about Telegram may be related?