Hi xuky, I am going out on a limb here because I have not only not seen Scaly-sided Merganser, 1st-w plumage is poorly described, and its not even illustrated in Reeber's HIG to Wildfowl. I am sure its a 1st-w due to a combination of various features including bill pattern, paler outer eye, short crest, pointed outer remiges and wing covert pattern, the latter presumably similar to other Mergus. I would sex it as a drake due to the discreet dark patches around the eye and on the rear crown.
And so my suggestion is a 1st-w drake Scaly-sided Merganser, but it would be good to get some input from someone who has first hand experience of the species.
Don't know how I missed this before, referring to Reeber Plate 69 p168, re ageing of SSM based on upper wing pattern like RBM (h, k, m & o). 1st-w drake (m) shows bar on greater coverts almost absent which seems to accord well with OP (image 4) which shows black tip to 2nd (?) outermost + 2 inners with small black tips.
Thank you Grahame.
I always thought that the juv duck are very close to the female in its plumage, so I was confused about its pattern of wings. and I thought it maybe have the blood of a Common Merganser , but other than that, there was nothing in line with a Common Merganser.