Stop Photos Auto Syncing iCloud

I'm looking to sign in to my Apple ID on a second device.

There's an app I want to use in unison with my main device.

On the second device, there's a bunch of videos / photos.

Some of these vids / pics are already stored in my iCloud, from having previously used this second device with my Apple ID.

But there's some vids on this second device that I don't want to be uploaded on to my iCloud.

I have recently deleted them from my iCloud - but they're still stored on this second device.

If I sign in to my second device, will all the vids / pics start syncing with my iCloud?

If so, how do I stop that?