Q: iCloud Backup Issues

I have iPhone 5s, iOS 9.3.5 (latest), 16 GB. I have additional storage bought on iCloud Drive worth of 50 GB.

Currently my iPhone no space left. Usually the phone backs up the photos, documents, app data on icloud freeing up the phone. Lately i have not been able to back up the data on my phone on the iCloud. Following things i have tried

1) Connected to WiFi, Settings -->i Cloud -->Back up--> Back up Now

It updates the last back up taken. However it doesn't reflect the iCloud menory consumption

2) I have cleared out un necessary apps, which were occupying space. (so please dont recommend this)

3) Need to understand, once the phone backup does it always free up memory? How about the memory consumed by apps? Once backed up does it free up the

The main memory on iPhone is consumed by Photos app (close to 6.6 GB), which i want to back up on iCloud and free up the phone memory.

I have been unable to achieve this. Any help from the forum appreciated. Thanks