Will it harm the iPhone 12 to use the SIM card from iPhone 11?
Will it harm the iPhone 12 to use the SIM card from an iPhone 11?
iPhone 12 mini, iOS 16
Will it harm the iPhone 12 to use the SIM card from an iPhone 11?
iPhone 12 mini, iOS 16
No, it won't harm it. However, you won't get access to your carrier's 5G network.
See if they are compatible: Learn which size SIM card your iPhone or iPad uses - Apple Support
If they are: Remove or switch the SIM card in your iPhone - Apple Support
Will it harm the iPhone 12 to use the SIM card from iPhone 11?