If I buy a new iPhone , would my hidden photos and videos be transferred automatically to the new phone?

If you sync them to iCloud currently they will sync from iCloud to the new phone.

If you don’t sync them to iCloud but back them up to either your computer or iCloud (which is not the same as syncing to iCloud) when you restore that backup to the new phone they will be transferred.

See→Transfer data from your previous iOS device to your new iPhone, iPad, or iPod touch - Apple Support