Getting my data back

Sign into the new iPhone 12 using the same Apple ID you used on the iPhone 11. In the set up of the phone, if you had a backup from the iPhone 11, you'll be given the option to restore the new phone from the backup you made (if you made one) on the iPhone. See this link --> Transfer data from your previous iOS device to your new iPhone, iPad, or iPod touch - Apple Support

If you didn't ever back up the iPhone 11, as long as you are using the same Apple ID, you can download all your apps you had from the App Store, by going to the Purchased tab in the App Store and all your apps under that Apple ID will be shown with a cloud symbol next to them. Tap the cloud symbol and apps will download.

For things like photos, etc., after signing into the new phone with the same Apple ID, go to Settings > Your Name, iCloud, Media & Purchases. Tap iCloud. Turn on all the services, such as Photos, etc. and if you had synced them with iCloud, they will download to your new phone.

If you didn't back up, nor sync with iCloud and you erased your phone without backing up data, your data is gone.

Make a backup of the iP 12 and restore that backup into the iP 11.

I think the OP is suggesting the erased the iPhone 11 and want to restore the data from a phone they no longer have to an iPhone 12.