If my iPhone was stolen, how can I get my Notes?

If your Notes were synced with iCloud, you can use an internet browser on any device and log into www.icloud.com and go to the Notes app there. Also, if you set up iCloud Notes on another device, the Notes will automatically sync with it. See: Set up iCloud for Notes on all your devices - Apple Support

If they weren't synced with iCloud but you have a backup of the phone, you can use the backup to setup a replacement device. See: Transfer data from your previous iOS or iPadOS device to your new iPhone or iPad - Apple Support