Most people think of backups only after something happens: the phone fell in water, was stolen on the bus, or was hacked and locked. That is when you find out whether years of photos, messages and documents still exist somewhere or are gone for good. Setting up a backup takes about fifteen minutes, and it is better done today.
The rule IT people recommend is called 3-2-1: three copies of important data, on two different kinds of storage, with one kept outside the home. For most people that means the data on the phone itself, a copy in the cloud, and another copy on an external drive or computer. That way, even if the phone disappears and there is a problem with your cloud account, a copy remains.
On iPhone: go to Settings, tap your name, then iCloud. Make sure iCloud Backup is on and Photos sync to iCloud Photos. The free storage is limited, and anyone with many photos will probably need to pay for more. On Android: on most devices backup is in Settings under Google, then Backup. For photos, Google Photos offers automatic backup. Free storage is limited here too and shared with Gmail and Google Drive.
WhatsApp is a separate story, because messages are not always included in the general backup. In the app, go to Settings, then Chats, then Chat backup. On iPhone the backup is stored in iCloud, on Android in Google Drive. Turn on automatic daily or weekly backup, and turn on end-to-end encrypted backup so nobody but you can read it. Just don't forget the encryption password.
And the third copy: every few months, move important photos and documents to an external hard drive or computer. External drives are inexpensive, and this is the copy that will save you if your cloud account is hacked or locked. A final tip: check occasionally that the backup actually works. Sign in to your cloud account from a computer and make sure last month's photos are there. An untested backup may be a backup that doesn't exist.
