bojanz, thank you for helping out. I have read the Backoffice page and the documentation here (which still describes the "old" way). I understood that the "old" UI was disabled in favour of a "new" UI.
What I didn't realise is that it wasn't replaced. Backoffice changes both the commerce admin menu structure and the wording. I think this should be communicated more clearly since these changes are pretty significant.
Adding to my confusion, Backoffice doesn't display product types (variations) that are not referenced by a display, what made me raise my eyebrows even more as I couldn't see the products I had created. At that point I felt kind of lost.
In the end the "new" concept makes sense and of course is beneficial and the right direction. But for those who aren't directly involved in the module development, it is hard to understand what's going on due to limited information.
This is my experience (for those who have the same questions): In the admin menu, the product configuration has been separated from the product management.
Creating a new type/variation is now in
Shop > Configuraion > Product variation type > Add product variation type
Adding a new product within a type/variation:
Shop > Products > Add products > (choose from one of your types/variations)
Creating a new product display type
Structure > Content types > Add content type
Adding a full product (display + variation/type) inline:
Content > Add content > (choose your product display content type)