How do I share the extra memory I bought for my IPhone with my IPad??

The extra storage is not on your phone, an iPhone or IPad has a fixed storage size and cannot be increased.

You bought iCloud storage which is used for syncing. If you log into both devices with the same account, you will have access to that iCloud space on both devices.

If you got memory like when you bought it like say you bought the 256 GB iPhone then you cannot share that but if you are talking about iCloud storage then it should already be on your iPad if you are signed in with the same Apple ID on your iPad as you are your phone. You can check to see if your iPad has the iCloud storage by going to settings < click your name at the top < then click iCloud and it should show your storage amount.

If you only use 200gb or less of that you can switch to the cheaper 200GB plan iCloud+, if you have family you can share it with them see this article Share iCloud+ with your family - Apple Support

[Edited by Moderator]