It cannot fill in passwords on an iOS app unless the developer of that app has done some legwork to integrate with iCloud Keychain.
It does not work with any third-party browsers on OS X or iOS. The short version is that iCloud Keychain does a good job of automatically entering passwords in websites on Apple's Safari browser, both with iOS devices and Macs. It works best as a complement to a complete service like 1Password or LastPass, but it just isn't convenient and robust enough to act as a standalone password manager. Ultimately, iCloud Keychain can be put to good use if you've carefully examined what it does well and doesn't do well.
Important functionality is hard enough to find that it may be effectively hidden from the average user, particularly on iPhones and iPads. iCloud Keychain does accomplish the most basic things you'd expect a password manager to do, but it often does so in an awkward manner. There’s also a handy search option in the top right corner of the preferences window.In the latest versions of OS X and iOS, Apple's new iCloud Keychain provides one of the most important pieces of functionality for security-conscious users: a password manager. Use the Add button to create a new entry. If you’d like to delete multiple items at a time, hold the command key while clicking items, then use the Remove button in the bottom right corner. Double click a field (user name, password, etc.) to make edits. Now you can click any item to view the password or make changes. If it’s not already selected, click Passwords at the top of the preferences window and enter your Mac’s password or use Touch ID.