Elgato has released Stream Deck Mobile, an app that turns iPhones into a broadcast controller for videos on YouTube, Twitch, Mixer, and elsewhere.

After configuring via the desktop version of Stream Deck, Stream Deck Mobile can include up to 15 action buttons. These trigger functions like audio, lights, going live, launching websites, switching cameras, and popping up onscreen text.
The apps effectively duplicate Elgato's hardware Stream Deck. Some other compatible software and services include Twitter, VoiceMod, Spotify, Philips Hue, OBS Studio, Streamlabs, and Nvidia Share.
The desktop/mobile pairing process is handled by using an iPhone to scan a QR code. Once that's complete, both devices must remain on the same Wi-Fi network.

is free to download for iPhones with iOS 12.2 or better, but full use costs $2.99 per month or $24.99 per year. Desktops must be running at least Stream Deck 4.3 on macOS 10.13 or Windows 10.