Welcome 
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…and 
for spending enough time here to post more than on image… but TBH… you images doesn’t show your actual problem… as shiny as they are… a simple shaded view and also in editmode would show us the geometry. Also i’m unsure about the word "mask" here… especially with the last image and the black lines…
…according to image 3 it seems you have some proper geometry for this weird kind of… techno whisker (?) but then you have some very thin stroke on you image texture and also use this for tranparency ??
So at least i don’t quite understand and think you have to elaborate this some more…
Why don’t you start with the video method. 
It’s different from the design you’re trying to make, but the way you start it is similar.
Character Sculpts Reference
https://www.youtube.com/@yansculpts/videos
mask in this context is talking about the sculpting tool.
in the tutorial (4:30) https://www.youtube.com/watch?v=3Dw8FwCi5aY&t=355s here they put a circle and then invert the circle to drag that part.
the weird lines are simply grease pencil.
the pink part is where I want to add the neck but the tutorials for these aren’t what I was looking for
Ohh… well… IDK if sculpting is the right path for a beginner… to begin with… !??
In sculpting you do a lot of things and already have to know about them… like for example having the sense of correct forms etc… like the skill needed to be able to make some recognizable drawing (not a masterpiece)…
In polygon modelling this is done step by step adding some more geometry and getting this into the right position…
…but of course i don’t want to hinder you. And maybe first follow something not so complex like @oo_1942 suggest… ? (Or even simpler ?)
maybe sculpting might not be what I’m looking for. A better idea might be to draw the neck I want and then extrude what basic form it’s from. However the original .krz files I used for this model’s ref are gone so maybe take two renders of thee face and draw the neck I want
As someone who started learning 3D with sculpting (and still can’t poly-model), I would say that hybrid approach is my favourite. Use modeling for anything that is too difficult to sculpt, and vice versa. Remesh/retopo afterward depending on your needs.
Regarding mask placement: there is no recipe for this, I’m afraid. You paint it… somewhere at the bottom of the skull. If it looks too weird, press “undo” then try again 
Extrude the neck, then Grab/Add clay/Smooth/Scrape/whatever until it looks ok.
Or you can add a cylinder, position it where the neck should be, join to the sculpted object and Remesh.
Or do what you suggested: extrude the form and then sculpt it.
There are many methods, but the only way to know which one you should use… is to try them all, as each will find some ways more challenging then others.