Howdy,
When you use iCloud with various devices it can keep things like photos, calendars and such synchronized between them.
If you don't want them synchronized you would turn off that function for the data type (like Photos) that you want to keep separate.
See Set up iCloud on your iPhone, iPad, or iPod touch
Go to Settings, tap [your name], then select iCloud. Choose the apps—like Photos, Contacts, Calendars, and third-party apps—that you want to use with iCloud.
Set up iCloud on your other devices to keep your content up to date everywhere.
You would turn off Photos > iCloud Photos, and whatever else you want to keep separate.
Note you only have to do this with one of your iPhones if you want the other to still stay in sync with your Mac or other devices.
ivan