How to sync app on ipad to iphone when app doesn't appear in icloud?

Arandel wrote:

Can I transfer an app purchased on my ipad to my iphone?

No. That isn’t possible.

the app doesn't appear under "apps using icloud."

That is not how you would download and app that you have already purchased. Launch the App Store App. Tap on your account icon in the upper right corner. Tap on Purchased. Find the app in the list and tap on the download cloud icon.

You must be using the same Apple ID that you purchased the app with originally in order to download it again for free.

You’re welcome, but if we are going to be technical, I’m not wrong. You cannot directly transfer an app from one device to another. You can download it from the cloud. I described how to download a previously purchased app.