which iCloud app is best for keeping photos stored on my 50GBs of purchased data space?

Are you syncing photos to iCloud using the iCloud Photo Library? If you are syncing the iCloud Photo Library, and your phone dies, when you get a new phone, sign into Settings/iCloud using the same Apple ID as the old phone. Then turn Photos syncing on and your photos should download.

If you are not syncing photos to iCloud, to get your photos back, you will have to restore the backup of the old phone to the new phone.

Hi - you would use iCloud Photo Library and the Photos apps on your devices for your photos. Here's some more info that explains it: iCloud Photo Library - Apple Support

If you are syncing photos per the above suggestions, they aren't included in your iCloud backup or iCloud Drive.

Backups – Choose Method

Back up your iPhone, iPad, and iPod touch

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iCloud backup is a backup system for IOS devices, the others are not backup at all. Photos is the app you would use to store photos, Time Machine is the way to back them up (on a Mac)