How do I restore from a iCloud backup to new iPhone?

Details here:

Use iCloud to transfer data from your previous iOS or iPadOS device to your new iPhone, iPad, or iPod touch - Apple Support

If you have already set up the phone and bypassed the option to transfer data from an iCloud backup, you will need to erase the phone and start over again with a new setup.

To restore your replacement iPhone from an iCloud backup, you can follow these steps:

1. Turn on your new iPhone: A “Hello” screen should appear.
2. Set up manually: Tap “Set Up Manually”, then “Restore from iCloud Backup” and follow the onscreen instructions.
3. Sign in to iCloud: Use your Apple ID and password.
4. Choose a backup: When asked, select the backup you want to restore from. Make sure it’s the correct one by checking the date and size of each.
5. Stay connected: Keep your device connected to Wi-Fi and wait for the restore process to finish. Then, complete the rest of the onscreen setup steps.
6. Keep your device plugged into power after setup: This allows content like your photos, music, and apps stored in iCloud to automatically download back to your device.

If you’ve already set up your new device, you need to erase it before you can use these steps. To erase your device, tap Settings > General > Transfer or Reset [Device] > Erase All Content and Settings. This erases all data on your device. After you erase it, your device restarts and the “Hello” screen appears.

If you do not have an iCloud backup, you can use this source: Use Quick Start to transfer data to a new iPhone or iPad - Apple Support

I hope this helps!