where do you go after that ?

I was wondering where he went! :O

You're correct. If you unlock the last ability in the prosthetic tree, it speaks of the sculptor and his bodycount, mentioning the "flames of hatred" Also if you speak to the info merchant at the shrine and choose the "talk" option after the sculptor leaves, he'll give you a bit more insight.

Another neat way of telling is the demons left arm is missing/made out of fire.

worst advice on the page

Yeah no. This is bad advice. If you are under his feet many of his attacks miss and you can parry his leg stomps.

Are you 100% sure that the confetti deals increased damage to him? I've heard the opposite from other sources. I'd like to know before farming more confetti.

^ Anon, yeah I can confirm it does more damage. But the shield is way more worth it for your emblem use.

It definitely is the sculptor. The left arm is missing, replaced by the fiery appendage instead (Sculptor is the only other character we know who is missing their left arm. He's affected by the finger whistle (Belonged to the other shinobi he lived with who was killed by the guardian ape), and when you kill him it says "Wolf, Thank You..." The sculptor, Owl, Emma, and Kuro are the only characters who refer to Sekiro as "Wolf" and we know where Emma and Kuro are at that point in the game you've already killed Owl. The final bit of evidence is the Old Hag who will comment after you kill the Demon that Sekiro knew him from before. Sekiro will say he did not to which the hag will respond "It's probably best that you don't know." She then mentions how he failed to become Shura and turned into a demon instead. Sekiro is leaves a lot less ambiguity than Soulsborne does.

A bonfire that I think is at the start of Ashina Castle will automatically teleport you into the area.

In travel menu Ashina Outskirts at the bottom it shouldve created a new Idol