You do it on the iOS device you’re trying to set screen time limits on. If you don’t own an iOS device, you’re not going to be setting limitations on an iOS device.
Parent mentions Google family link, whih can be setup and managed from litteraly everywhere. There's an iOS app and a fully functional website, so you can have no other android device than your kids, and still control everything.
Your kid can be visiting their grand parents and ask you to authorize some purchase or extra screen time, you'll be able to do it from your Windows computer.
That's the standard iOS' parental control is compared to.