How do I add something to favorites or bookmarks on iPhone 12 Pro Max?

If you are asking how do you add a YouTube video to your Safari app's Favorites list, then when you select a video, look at the bottom of the display for a icon that looks like a box with an up-pointing arrow. Tap on it, and then, tap on "Add to Favorites."

Ref: Bookmark a website in Safari on iPhone - Apple Support

If you want to add YouTube as a shortcut in Safari, so that it is on the front page. Then open Safari, visit youtube.com tap on the "Share" button in the bottom line in the middle (Square with Up Arrow) - scroll down and select Add Bookmark. YouTube will be available as a shortcut on the first page when you open Safari.

YouTube also has its own app, which you can download and use for free in the App Store:

• Download YouTube for iPhone

You open Safari by tapping on the Safari icon. In the search field, you can search for web pages - for example, you can write youtube.com and tap Go. Then YouTube will open and you can tap on the videos you want to watch or search for them.

You can see how to use Safari on your iPhone here:

Browse the web using Safari on iPhone - Apple Support

In order to use Safari, you need access to the internet, either via Wi-Fi or Mobile Data.

You can see how to connect your iPhone to Wi-Fi here:

Connect to Wi-Fi on your iPhone, iPad, or iPod touch - Apple Support

You can see how you get and use Mobile Data here:

Use cellular data on your iPhone or iPad - Apple Support