iCloud storage

I’m confused about Apple iCloud.  Correct me if I’m wrong and set me in the right direction

When I buy an Apple product I have a small amount of cloud space, I can buy more for a small fee. 

My confusion is what this space is and what use is it! As I understand it it if I create a document on my iPad it is automatically stored on iCloud. But, and here is one of my difficulties, if I delete a file from the iPad it is automatically deleted from the cloud and all other Apple products I have eg my iPhone.

So let’s say I want to delete a file from my iPad: text, music, video, whatever, but still have access to it can I keep it up there in iCloud, if not what do I have to do to hold on to it. I do appreciate there are other cloud backup services out there but surely Apple have something where we can store files safely and long term!