Write and draw in documents with Markup on iPhone - Apple Support

Save and edit PDFs to the Books app on your iPhone or iPad – Apple Support (UK)

Use AssistiveTouch on iPhone - Apple Support

Create and edit events in Calendar on iPhone - Apple Support

Send and reply to messages on iPhone - Apple Support

Use iPhone as a level - Apple Support

Edit photos and videos on iPhone - Apple Support

Write and draw in documents with Markup on iPhone

Find files on your iPhone or iPad in the Files app - Apple Support

How to Use Markup on an iPhone, iPad, or iPod Touch

Draw, annotate, and use Scribble in Pages, Numbers, and Keynote - Apple Support

How to Draw in Emails on iPhone & iPad

How to Set Up Your Signature in Apple's Markup & Make It Easy to Sign Forms on Your iPhone « iOS & iPhone :: Gadget Hacks

Mark up files on Mac - Apple Support

Use Live Text to interact with content in a photo or video on iPhone - Apple Support