Everything is UX. UX starts with underlying data analysis - see Taxonomy and Card Sorting on the linked page. It continues through the development process. It doesn't directly comment on service implementation, but it does help specify what those services need to deliver

I think so. Some examples of places where this has impact but is often overlooked could be when you change car brand. Or if you go from one phone OS to another. The UX features will be slightly different, and there will be a learning curve.

Pulling down to update on feeds/pages/apps is as natural as clicking icons by now.

Co-production is a form of design, no? You design within constraints and it should be happening in tandem with the development of the system so they both feed into one another.

Yes you can design different experiences on top of the service/system. Customer support, packaging, onboarding process, flows etc.