How to restore missing iCloud Notes on my iPhone 12?

First, verify the notes you want to sync to your iPhone are the ones that you see in iCloud.

View notes and locked notes on iCloud.com - Apple Support

If iCloud is showing all your notes, then continue by going to Settings > [name on top] > iCloud > Notes, and turn the switch off if it is on. If it is already off, then simply turn it on and wait for them to download and you are done. After shutting the switch off, then choose the option to remove the notes from your device as you currently are not seeing any notes from iCloud there anyway. Most people restart their phone at this point, but you may just be able to wait 30 seconds and turn it back on. Your notes that you viewed in iCloud will now sync back to your iPhone.