erase all content and setting on a iphone

Yes, they will be deleted from your iPhone (but you can still retrieve them if they are stored in iCloud).

How to erase your iPhone, iPad, or iPod touch - Apple Support

Restore your iPhone, iPad, or iPod to factory settings - Apple Support

Yes, it deletes everything from the iPhone only so you’ll want to make a backup first. It does not delete anything synced to iCloud, tho.