How do I remove my iCloud from my old iPhone without deleting it from my new one?

So I used to have an iPhone 7, I recently upgraded to a iPhone XS or XR i don't know. And when I set up the new phone my old one just send over all the info and I didn't have to re-download any apps it just did it all. Now I want to use my old iPhone as a business phone so I bought a new SIM card and yet when I use the old phone it still has all my old stuff on it and when I try to delete photos from the old phone it deletes in my new one as well. How do I disconnect them and erase everything from my old one without losing it on my new one?

All you need to do is log out of iCloud on the old phone, and it will stop syncing. If you really want a clean slate, erase it and create a new Apple ID for business purposes.