iCloud back up question

Good day,

I would like to back up my iPhone to the iCloud to free up storage space on my phone.

After backing up I would like to delete some photos and keep some on the phone.

I was told that once I do this photos on the iCloud would also be deleted.

i wanted to know if this is true.

is there a way to back up the phone to the cloud, free up storage space in the phone without losing what is in the cloud?

thanks in advance.