How can I upload my photos to iCloud and free some space on my phone?

You may sync Photos to iCloud Photos. Note: You cannot delete Photos from iPhone which will delete it from iCloud too, if you use sync.

Select Optimize iPhone Storage to save space on your device. With Optimize Storage, smaller, space-saving photos and videos are kept on your ...

Manage your photo and video storage - Apple Support

However for upload to iCloud Drive --> Access iCloud Drive files from your iPhone and other devices ...

Set up iCloud Drive to keep your files up to date across your devices.

On your iPhone, iPad, and iPod touch with iOS 11 and later, you can access your files from the Files app when you have iCloud Drive set up. The Files app makes it easy to share your documents, collaborate in real time on your Pages, Numbers, and Keynote projects, and manage everything that you store in iCloud Drive.