Q: Tranfer pictures from Photos app to iCloud Drive folder

Go to Setting > iCloud & Turn ON Photo Stream & iCloud Drive in Settings

You can't "transfer" your photos to iCloud.

You can sync them, but they will still be present on both your phone and iCloud.

To reduce the amount of storage in use on your phone, you need to import your content to another device like a computer.

Import photos and videos from your iPhone, iPad, or iPod touch - Apple Support

iCloud Photo Library is meant to sync photos across your Apple devices. While you can set it to optimize photos and reduce some space on your phone, it is not ideal for this. Please read these FAQs before you enable iCloud.

iCloud Photo Library FAQ - Apple Support