Google Cast, or Chromecast Built In as they're now calling it officially supports Android, iOS and the Chrome browser (on Windows, macOS, Linux and Chrome OS)
There are 3rd party apps that extend functionality like Soundcast that thewinelake mentioned, although that's no longer developed, but there are plenty of other free alternatives like mkchromecast for example, or the commercial Airfoil, which now supports CCA, plus countless other apps on Android and presumably iOS, but it's not the best way to use them IMO.
All these third party apps just turn Chromecasts into AirPlay clones of sorts, in that you're then using a secondary device as a server, rather than just as a remote.