Email QR editor.

Their default email app opens with the To address pre-filled. If you've added a subject or body, those fields are pre-filled too — they just tap Send.

Yes. Fill in the Subject and Message fields in the editor and they'll be included in the mailto: link encoded into the QR code.

No — any email app that responds to mailto: links will work, which covers Gmail, Apple Mail, Outlook, and most other clients on both iOS and Android.

Yes, as long as an email app is installed and set as the default mail handler. This is true for virtually all smartphones out of the box.