TL;DR
On iPhone, Safari and many PDF views let you capture the whole page, not just the part currently visible on screen.
How to capture it
Take a screenshot as usual by pressing the side button and volume up button at the same time. Then tap the preview in the lower-left corner before it disappears.
Choose full page
At the top of the editor, switch from "Screen" to "Full Page". You will see a long page preview on the right, and you can crop or mark it up before saving.
Save it where you need it
Tap "Done" and save the file, usually as a PDF, to the Files app. This is useful for instructions, order confirmations, travel details, receipts, or longer articles you want to keep without searching again.
Good to know
This works best in Safari and PDF documents. Some apps will not show "Full Page" because they do not hand the full document view to iOS.