Is iCloud a Backup?
Is iCloud backing your phone up? The answer is: maybe.
Apple iCloud itself, contrary to popular belief, is not a backup. Rather, it's a sync. This means that it exists at the same time as the files on your phone do.
This also means that, if you delete any of those files, they'll be deleted from iCloud as well. Unfortunately, this leaves many iPhone users with deleted files, mistakenly believing they've been backed up.
Here's where the "maybe" comes in: iCloud Backup does exist! You just have to enable this option by heading to Settings > [your name] > iCloud > iCloud Backup. If you're using iOS 10.2 or earlier, go to Settings > iCloud > Backup.
According to Apple, "When you set up iCloud, you automatically get 5GB of storage. You can use that storage space for your iCloud backups, to keep your photos and videos stored in iCloud Photos, and your documents up to date in iCloud Drive." Even if you do enable a backup, you're paying a monthly fee for your storage (beyond the initial 5GB). If you stop paying, you lose it.
This is why permanent solutions for your photos & videos are necessary. FOREVER is that solution. Storing your files in the cloud makes sense - you just need to be sure your memories are truly backed up.