Screenshot

If you want to take a screenshot of an entire web page - just take a screenshot as usual. Side Button + Volume Up. When you see the screenshot, select Full Page at the top.

After that, you can choose to share it, or save the image as a PDF file.

You can find more useful information here:

Take a screenshot or screen recording on iPhone - Apple Support

What a long screenshot is not clear to me.

Are you referring to Screen Recording?

Record the screen on your iPhone, iPad or iPod touch

1. Go to Settings > Control Centre, then tap the Add button next to Screen Recording.
2. Open Control Centre on your iPhone or on your iPad.
3. Tap the grey Record button. ... 
4. Exit Control Centre to record your screen.

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Record the screen on your iPhone, iPad or iPod touch