iCloud syncing photos - what is the effect?

I was going to buy a flash drive with a USB-C connector and a Lightning connector, so I could transfer photos back and forth between my iPad Pro (3rd gen) and iPhone 13 Pro Max. But then I thought about iCloud. Reading about it says it will sync photos across the devices. But here’s the thing. My iPhone and iPad have completely different pictures in the Apple Photos app. I want to keep them all. I don’t want to sync across the two devices and end