What makes a good ecommerce product page?

It answers the objection before it is raised. Price, delivery date, size or fit, and what happens if it is wrong — visible without scrolling on a phone.

Why the answer is what it is

Photographs do most of the remaining work, and the useful ones are scale and context rather than the white-background hero. Everything else on the page is decoration for a decision already made.

What this looks like in practice

Most design decisions on a store are really navigation decisions, and navigation is what decides whether somebody finds the thing they came for.

A product page has one job. Everything on it that is not helping somebody decide is in the way.

Consistency beats cleverness. A customer learns your store once, and every screen that behaves differently costs them.

Where a mobile app comes into it

The same screen designed several ways is the point: a catalogue of four hundred lines needs a different home screen from one of forty.