hello chia rac,
iCloud is greatly misunderstood. It is syncing storage and anything in it is also on all devices used with that Apple ID too. If you want to reduce photos use of your phone storage you can select to optimize those copies and only keep full quality ones in iCloud. The link below has the general information you need about different kinds of "storage", how to get more, or how to better manage what you have:
What's the difference between device storage and iCloud storage? - "Learn the difference between your device and iCloud storage, ways to manage your space, and where to buy more if you run out." Click here --> https://support.apple.com/HT206504
Click on this link to learn how to optimize the storage actually on the device itself --> "How to check the storage on your iPhone, iPad, and iPod touch" - https://support.apple.com/HT201656 - "If your device is low on storage, iOS automatically optimizes its available space while installing an app, updating iOS, downloading music, recording videos, and more. To make more storage available, iOS can remove some of your items, like streamed music and videos, files in iCloud Drive, and parts of apps that aren't needed. It also removes temporary files and clears the cache on your device. iOS only removes items that can be downloaded again or that aren't needed anymore."
Optimization specifically with reference to "Set up and use iCloud Photos" - https://support.apple.com/HT204264
--> Save space on your device
iCloud Photos keeps all your photos and videos in their original, high-resolution version. You can save space on your device when you turn on Optimize Storage.