On Android we support NIP-55 remote signers like Amber - https://github.com/nostr-protocol/nips/blob/master/55.md
On iOS there isn't really a good NIP-46 remote signer available. I know jb55 (npub1xts…kk5s) is thinking of adding this to damus (npub18m7…q955) which would be amazing and we would support it straight away.
Not sure if Alby (npub1get…0nfm) has plans for a mobile NIP-46 signer.