Hello Vikas,
Good question. One thing I would like you to know and that is that iCloud storage is NOT an extension of the storage on your iPhone. The iCloud storage is used in parallel with your iPhone storage. Think of iCloud as a synch service. The data that you have on your iPhone will be mirrored on iCloud provide that iCloud does not run out of space.
In your example, You have 50GB of iCloud storage space and majority of that space has been taken up by photos. Your iPhone X has 256GB of storage space and most of it is unused. How can you transfer pictures from iCloud to your iPhone?
If you have your iPhone X setup on iCloud properly then the synch process must have already begun between your iCloud and your iPhone X. Do you have Optimize Phone Storage active on your iPhone in settings? If you do then iCloud is storing low resolution version of your photos on your iPhone and saving more space. Can you double check that setting on your iPhone?
Open Settings ➔ Photos ➔ Download and Keep Originals - Make certain that this is Checked.
Make certain that iCloud Photos is SET to ON on the same page in settings.
This should begin synching and downloading original resolution photos on to your iPhone.
Axel F.