When you open a product page on a phone, the visible area before any scrolling is small. For a first e-commerce site, the most common mistake is trying to show everything at once: a banner, a long product name, several badges, a description snippet, and maybe a promotional strip. Each of these competes for the same few hundred pixels. A practical starting point is to decide which single piece of information your visitor needs first to make a decision. For most product pages, that is a clear photo and the price. Everything else can move below the fold.
A useful test is to open your page on a phone you actually carry, not just in a browser preview resized to a narrow width. Real devices have different aspect ratios, and system bars or browser chrome eat into the visible area. If your add-to-cart button requires scrolling on the phone you use daily, a significant number of visitors will have the same experience. Moving the purchase action higher, or making it a fixed bar at the bottom of the screen, is a straightforward fix that does not require design expertise.
Text on the first screen should answer the question a visitor is likely asking at that moment. For a product page, that question is usually "what is this and how much does it cost." A short product title and the price fulfill this. Longer descriptions, specifications, shipping details, and reviews all serve different questions that come later in the visit, so they belong lower on the page. If you find yourself adding explanatory text above the fold, it often means the image or title is not doing its job, and the better fix is to improve those rather than add more words.
One thing to watch as you iterate is that changes to the first screen affect what visitors see on every device, not just phones. A layout that works well on a narrow screen may look sparse on a desktop monitor, and that is acceptable. You can add information for larger screens later without removing anything from the mobile version. Starting with the smallest screen and expanding outward keeps the priority order clear and avoids the situation where you design for desktop first and then struggle to compress everything into a phone-sized view.