Regarding keychain for password filling, you can choose to disable it in the settings -> passwords -> options. It’s selected by default but if you tap it it will deselect it.
- The keychain is an essential part of macOS and iOS; if an application wants to securely save data then that is the interface it uses. This is not optional because a huge proportion of applications would break if it were removed.
- iCloud Keychain is the feature of iCloud that syncs your keychain to iCloud. iCloud itself is opt-in, and this particular feature of iCloud is opt-in as well, I think, or at the very least optional even if you use iCloud.
- Safari has a built-in password manager, like every other browser these days. This saves to the keychain, whether that’s synced or not.