Hello Mia,
Here is some great information âž” About Storage on your device and in iCloud - Apple Support.
Be sure to read the information in the link above ⬆︎⬆︎⬆︎
To summarize, iCloud storage is more like synch service to share data across all your devices logged into the same Apple ID. This function works for Messages, Photos and Videos, Notes, Calendars, Contacts, Reminders etc. If you are thinking of this service as an extension of storage to your iPhone then that is not the case. The way iCloud helps you save storage space on your device is by utilizing the OPTIMIZE STORAGE feature for Photos and Videos. What this does is store the actual resolution photos on iCloud and keeps a low resolution version of the same photos on your iPhone. It will only download the high/actual resolution picture when you demand it or when you double click on it to see it. If Photos and Videos take up most of the storage space on your device then this is the way to go. YES, if you delete a photo on your iPhone then it will be deleted on ALL your devices that are logged on to that Apple ID.
If you are looking for storage of your Photos and videos remotely and as an extension of your storage space on your iPhone then you can look into Google Photos which does exactly that. I believe that it is also free up to a certain amount of photos.
Axel F.