When you purchase an app on the App Store, it’s free to install onto every device that you own.
On each of your devices, just open the App Store app and navigate to the
Purchased tab (this is in the
Updates tab on iPhone). You can then install any app that you have purchased on any of your devices.
You will not be able to install your purchased apps onto devices that are owned by somebody else, your purchases are linked to your Apple ID.