How do access iTunes on my iPhone?

The iTunes referred to by the support document you came form is for computers only (Mac or PC).

On your iPhone you have several Apps that let you access different aspects of iTunes.

The iTunes Store App, as the name implies lets you access the Store, to buy media.

The Music App lets you access you purchased and synced music library.

The Tv / Videos App lets you access your purchased and synced movies, tv shows and other videos.

Do you mean your iTunes library?

1) Connect it to a computer running iTunes and sync your collection.

Sync your iPhone, iPad, and iPod with iTunes using USB - Apple Support - https://support.apple.com/HT201253 - detailed example: https://discussions.apple.com/thread/4090536?answerId=18860187022#18860187022

Sync your iPhone, iPad and iPod touch with iTunes using Wi-Fi - Apple Support - https://support.apple.com/HT203075

iTunes 12 for Mac: Sync iPod, iPhone, or iPad - https://support.apple.com/kb/PH19458 - including manual syncing

OR

2) Subscribe to Apple Music or Match and do it indirectly through iCloud.

iTunes for Mac: Use your iCloud Music Library to access all your music anytime, anywhere - https://support.apple.com/PH19612 - "If you’re an Apple Music member, or if you subscribe to iTunes Match, your music is stored in your iCloud Music Library, so you can access it at any time, wherever you’re connected to the Internet."

"iTunes" on a computer is an application for purchasing and playing media.
"iTunes" on a phone is used as an interface to the iTunes Sore (and has nothing at all really to do with this forum which is about iTunes on a computer).