A resized desktop browser is not a phone. It has no address bar eating the viewport, no thumb, no
The buy box pushed below a tall image gallery is the most common and most expensive mobile leak on Shopify product pages.
Testing in a resized desktop browser misses the address bar, the safe area, real touch targets and real network conditions.
Form friction costs more on mobile than anywhere else, and most of it is fixable through input types and autofill attributes.
Page weight hits mobile hardest, so speed work and conversion work are the same project rather than competing ones.
Some gap is normal — mobile carries more browsing and research traffic.
The full guide has the formats, the deadlines and the worked examples.