When you install the app on target devices, it called as "Client app" or "Target app". For example, it is installed on your child's device, or your employee's one.
And after installing that app, hide it, and then visit the website control panel to view the tracked data logs.
If you do not want to use the website, you can install another app on your device, called as "server app" or "monitor app".
You can install the client app on an Android device but can install the server app on your iPhone/iPad; or vice versa, you can install the client app on iPhone/iPad but you can install the server app on your Android device.
Both client app and server app are available on iOS and Android OS.
About target app: you can visit here for more details: How to install the mobile spy on target device.
Here we show how to use with server app: