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. Spatia Nostra 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 Spatia Nostra provided the link for. My particular favorite option, though, is Keychat. 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.