Why do my photos stored on iCloud take up so much phone storage?

I pay for 2 Ter of memory on iCloud+. I only have used 1/4 of that. My photos/videos are my constant burden and I thought iCloud was supposed to make it possible to put the heavy burden on my cloud not my iPhones internal storage. The settings are optimized for phone storage availability. My phones storage is full as soon as I log into my iCloud. New phone btw. Is this because my backups aren’t processing to completion? Also, why is that? It’s taking HOURS. Then fails. Internal memory full, every app is offloaded or deleted, phone is flipping out occasionally and screen flashes loading circle/gray screen and Lock Screen back and forth until I hard shut down. Had same issue with last phone. Wtf?