Exactly what do I do to buy 50gb iPad, iphone

Hello ~ Take a look here:

Upgrade to iCloud+ - Apple Support*

*How to upgrade on your iPhone, iPad, or iPod touch

1. Go to Settings > [your name] > iCloud > Manage Storage or iCloud Storage.
2. Tap Buy More Storage or Change Storage Plan.
3. Choose a plan and follow the onscreen instructions.

If you're on your iPhone, iPad, or iPod touch with iOS 14 or iPadOS 14 or later, tap here to upgrade to iCloud+. 

You will have to make your own appointment:

Genius Bar Reservation and Apple Support Options

~Katana-San~