transfer my photos to 200GB storage. My phone is full

iCloud storage does not add storage onto your phone. It adds storage into Apple's cloud storage system, here you can save backups and photos.

You could save space on your phone by keeping low quality/resolution photos on your phone but keep the high quality originals in iCloud.

Take a look at this help article for more info Set up and use iCloud Photos – Apple Support

Before enabling iCloud Photos on the device, you will have to do the following to ensure there's enough storage on your device -

https://www.youtube.com/watch?v=bnhuHSv_WrI