2 Apple IDs-How to merge purchases

I have 2 Apple IDs, on my ipad it has 2 shown ie. as my sign-in & the other for media & purchases. But, on iphone I only have the one ID (@icloud.com) for both. So, same purchases on both the ipad & iphone are not accessible. how do I merge purcahses or at least have icloud.com for sign-in and gmail.com for purchaes? Can't remember how I did that. I have 2 ipads 2018 & 2021 and iphone 12 pro max.