We categorize Nostr apps into web apps and native apps.

For web apps, Keychat uses extension login (NIP-7), with the seed phrase and private key stored in the phone’s secure element.

For native apps, NIP-55 and NIP-46 handle cross-app and cross-device logins.

Because of iOS restrictions, NIP-55 works only on Android. iOS can use only NIP-46, yet when the native app and the signer app run on the same iOS device, signature requests and signatures still have to be sent through a relay, which is less than ideal.