Moving files and photos to iCloud

My iPhone storage is full, however, I have thousands of photos and videos. I wish to buy extra iCloud storage so that I can transfer everything from my phone to free up space and to do that I’m thinking once I’ve synchronised I will then delete all photos and videos on my phone. The question is will everything on iCloud remain or as I delete them on my phone they will go as well. Please help and guide. Thank you