Photo storage

Hi, just upgraded and got my very first iphone 11. Im off to New York next month so i thought I’d better upgrade to iCloud storage to 200Gb so i could back up all my photos and videos to the cloud every night as i only have 64GB model. So knowing my photos would be up in the cloud if i ran out of space on my phone i could delete huge volumes off my phone but know they was safe in the cloud to download when i get home, but a friend said when I delete them from my phone they’ll delete from icloud. So is this correct? If I delete anything off my phone its deleted from icloud? If so whats the point ? I assumed that 200gb would be great but if i only have 64gb phone id only ever have 64gb in the cloud because it’ll just keep getting deleted. Help! It said on my photo upgrade storage so i did. Surly there is a way round this🤞🏻 Thanks for any advice.

iPhone noob.