Is there a public specification for the NFC part of the new-for-2020 App Clip? Is it just an External NDEF record? I'm asking as someone who is responsible for encoding NFC and RFID chips, not as an app developer.
As @Gereon said they are just URL's and for NFC they would be URI records and the NFC specification for URI's is available at https://github.com/haldean/ndef/blob/master/docs/NFCForum-TS-RTD_URI_1.0.pdf