Aegis is the best option for logging into native iOS apps for the time being. That said, there isn't much support for it from iOS clients. The only one I know it works with is Nostur. nostr:npub18sufer6dgm9gzvt8gw37x08dk8gxr8u80rh8f4rjv4m4u73wlalsygnz5q posted the link to the TestFlight for Aegis already.

Outside of that, there is Nostash, which is a Nostr signer extension for Safari browser, currently also in TestFlight, and which nostr:npub18sufer6dgm9gzvt8gw37x08dk8gxr8u80rh8f4rjv4m4u73wlalsygnz5q provided the link for.

My particular favorite option, though, is nostr:npub1h0uj825jgcr9lzxyp37ehasuenq070707pj63je07n8mkcsg3u0qnsrwx8. This is a chat client for Nostr that comes with its own web browser built in. When using Nostr web clients within that browser, Keychat acts as a browser extension quite seamlessly.