You may not have backed up your iPhone manually but if, as you say, you had iCloud backup turned on then at some stage in the recent past it would have backed up automatically.
The conditions for this are explained in How to back up your iPhone, iPad, and iPod touch with iCloud:
Follow these steps to set up iCloud Backup to automatically back up your device each day:
- Make sure iCloud Backup has been turned on in Settings > [your name] > iCloud > iCloud Backup.
- Connect your device to a power source.
- Connect your device to a Wi-Fi network.
- Make sure your device’s screen is locked.
- If you get an alert says that you don’t have enough iCloud storage space to complete the backup, follow the onscreen steps to buy more storage
When you set up your replacement iPhone and restore your data from the iCloud backup it will draw down the latest backup and you can continue from that point.