How to download apps you buy on iPhone to iPad?

Any Apps that you purchase are forever associated with your AppleID. As such, assuming that both your iPad and iPhone are signed-in with the same AppleID, compatible Apps can be simply downloaded/installed from the Apple App Store without having to pay again. You should note that not all Apps will necessarily be available or compatible with both devices.

If instead your devices are signed-in with different AppleIDs, you may be able to share the Apps using Family Sharing.

Family Sharing - Apple

Set up Family Sharing – Apple Support

Share App Store, iTunes Store and Apple Books purchases with Family Sharing – Apple Support

https://www.imore.com/how-set-up-family-sharing-iphone-ipad