im not sure how to exlain this but..in some vectors/vexels i saw, the diff shades of color or whatever are outlined it black to make it more..sharp? or whatever. and i was wondering how you could do that in photoshop.
an example of it is this..

thats probaly because they made the first layer completly black then did the reg colors ontop of it (beige n etc. )

i use illustrator so i'm not sure. in photoshop though, i think you can go to blending options and do stroke or for example the nose, you can just make the path, and right click and do stroke or outline? don't exactly remember.

That would be hard, because it's difficult to keep your black outline completely consistent using that method.

I suspect it has something to do with stroke.

i don't know how to do that in photoshop, but in illustrator you do use stroke.