about the artificial skin and flesh covering his cybernetic parts, they're made of a material called Cyberformium (Combat Cyborgs have them), which can perfectly replicate flesh and skin yet can turn into metal with a certain trigger (Hei has one on his gauntlets that can have him transform between his "civilian" form and his true form, kind of like a magical girl)
Incidentally, I wanted to share these three intro descriptions for my series (themes included). The first intro is for Seasons 1 to 4, the second one is for season 5 (the Stealth Force season) and the third one is for my sequel movie: