How can I purchase more storage for my iPhone 6 Plus

iCloud storage or internal storage?... you cannot purchase more internal storage only iCloud.

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.
4. Tap Buy 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 your iCloud storage. 

You can't

See options here on how to free storage on your phone:

https://support.apple.com/en-us/HT203097

Note that this will buy them iCloud storage but it will not add ANY storage to their iPhone as they asked. The should follow the link that elcpu provided.