Signal and chromium as well. I am not sure about Ubuntu, but to me it looks like they have a product/design org.

Blender would be another example. How are they run?

Blender is a foundation:

https://www.blender.org/about/foundation/

Interesting. So all of the examples have salaried design and product folks?

Meanwhile there's thousands of volunteering FOSS projects that are engineers only. Do product & design people not have passion projects? Or they do but are for some reason unable to collaborate with engineers (or vice versa)?

It's a complex problem. Design agencies and individual designers are blighted by the spectre of "spec work" [https://www.nospec.com] [https://creativemornings.com/talks/mike-monteiro--2/1]. The tl;dr of it is that business will approach agencies and demand they work on a product for free, of a a cost lower than the wok is worth in an act of good faith for the promise of fully paid work in the future. Except the work rarely comes. The upshot is designers are taught the value of their worth at college/university.

The other problem is the FOSS community itself. See the concurrent post about GIMP [https://news.ycombinator.com/item?id=32624055]. Opinionated devs that either don't see the value of design or having a designer. I know that this isn't true for all projects, but there a bad apples amongst the bigger, more well known projects (GNOME for example) that will spoil it for everyone else. Again, GIMP is a great example of this. As a result, most 'passion projects' lead to designers working on small projects that inspire them, which are rarely software related, ore they look to create something that can provide an income, either as a small second income or as a new business.

Canonical basically supports Ubuntu. (Over and above the Linux kernel and other components many of which are supported by many different companies (and individuals).