Mobile Is Not a Smaller Desktop
390px is a different operating mode: touch, native back, gestures, safe areas and attention change the product interaction model.
Responsive design is not a desktop screen compressed to 390px.
Mobile changes input, keyboard behaviour, browser chrome, native back, gestures, safe areas, viewport resize, attention and one-handed reach.
The information architecture can remain consistent while the interaction model changes. Hover actions may need to become explicit. Two columns may collapse, but their priority still needs redesign. A drawer must cooperate with back navigation rather than trap it.
A 390px check is not only a visual test. It asks whether the product is still understandable in a different operating mode.
Architecture in the Wild
Field architecture through live state, information, interaction and product-boundary decisions rather than abstract diagrams.