How to make an NPC face a player?

This is actually simple to do.

To find the distance between you and the NPC we much subtract your torso's position from the NPC torso's position. This will give you the difference between the points.

For example, if PartA's y is 9, and PartB's y is 4, if we do

print(PartA.Position.y - PartB.Position.y)

the output will show 5. If you measure the parts manually, you will see that they are five studs apart on the y axis (five studs apart from their centers, not their edges). Therefore subtracting positions gives you the space between them.

However, subtracting Vector3's still gives you three numbers. That's why we use magnitude. The official definition from the wiki is, Magnitude is a term used across mathematics that means several things. In vector theory, the magnitude of a vector is the length of a vector. Which is confusing. Basically, magnitude converts the three numbers we got earlier into a single number equal to the distance between the center of the parts. Although probably not technically correct, this is how it seems to work.

print((PartA.Position - PartB.Position).magnitude)

You can do the same thing to the torsos.

if (character.Torso.Position - NPC.Torso.Position).magnitude <= 10 then
    --do stuff
end

To actually make the NPC look at you is much easier. Just use the CFrame.new(startPosition,lookAt) constructor.

NPC.Torso.CFrame = CFrame.new(NPC.Torso.Position,character.Torso.Position) --Make the NPC stay in the same spot but rotate to the player.