KDE is an entire desktop environment, with a bunch of applications and even partnerships that have yielded a KDE laptop. Should Thunderbird have been able to collect more money than KDE itself, there might be something that KDE can learn from Thunderbird.
No, you can run most KDE apps on other systems, including Windows and Mac. I use Kate as my text editor on my windows work machine.
I used to be a KDE dev. We were largely volunteers, unlike a lot of other FOSS projects that had hired coders. The KDE e.V. funding largely went to server maintenance and helping students attend the annual conference (travel expenses! I benefitted from this a few times). Not sure if it's still like that. In my era, KDE could easily get by on less.
They have KDE connect and some other tools but I'm not aware of how popular they are even for Linux users. A lot of it is duplication of other already popular tools available. And their bittorrent client sucks in my humble opinion.
I would expect for most users KDE is just another window manager for Linux. Secondly it's not even the most popular. So I would expect its user numbers to be much smaller.
Thunderbird is probably one of the single most used OSSs out there. I didn't even know any other mail client for Windows for a very looong time. Besides Outlook that is, which is quite expensive... Well and the Windows Mail app, which looked horrible to me before even logging in (and also came out after Thunderbird iirc)