On iOS there’s no truly “invisible” install of Eyezy unless you either jailbreak the device (which in itself is a huge red flag and easily spotted) or enroll the phone in a locked-down MDM/supervision setup via Apple Configurator. Even then a savvy user can open Settings → General → Profiles & Device Management to see any profiles you’ve pushed. In short, on a stock iPhone you can’t slip in a background agent without leaving tell-tale footprints.
On Android it’s a bit easier to hide an APK, but it still shows up under Apps & Notifications or flags massive battery/wake-lock usage. And Android’s looser sandbox means you’ll see odd permissions alerts or performance drains.
If you just need basic location or screen-time monitoring on iOS, I’d lean on Apple’s built-in Family Sharing + Screen Time. You get real-time Find My, app restrictions and usage reports—all fully supported by Apple and impossible to uninstall without your passcode. Much more reliable and private than a third-party stealth app.