how to transfer purchased items from iphone to ipad?

You redownload them from Apple:

Redownload apps and games from Apple - Apple Support

Redownload music - Apple Support

Redownload TV shows and movies that you purchased - Apple Support

For things on a computer, it may be possible to transfer some (not apps) by syncing.

Sync your iPhone, iPad, or iPod using your computer - Apple Support

If both devices are using the same "Family Account" or are using the same AppleID, purchases should be showing up. There might be a delay based off of internet connection and Apple devices "talking" to each other. It is what it is.

If the devices are not using the same "Family Account" or not using the same AppleID, then some account sharing needs to take place.

Check here for detailed assistance.

Set up Family Sharing on iPhone - Apple Support

Your unhidden purchases should show on any device where you're signed into your Apple ID. See Hide and unhide music, movies, TV shows, audiobooks, and books - Apple Support to unhide hidden purchases if needed. If you want to copy over non-purchased content then it might need to be reclaimed to a computer first. See Recover your iTunes library from your iPod or iOS device - Apple Community if that would be useful.

tt2

This part stumped me a few years ago!

I couldn't understand why I had to "repurchase" items that I KNOW I had bought at some point in the past!