If you are on a 'sliding scale', you lie somewhere between a range of values. Imagine a drawn line between '0' and '10' with '0' being an extreme introvert and '10' being an extreme extrovert. The text is saying introverts and extroverts are on a 'sliding scale' where everyone's position varies a little. I could be an introvert at '3' and someone else could rank a '4' as they are slightly more extroverted. Someone at '5' on this imaginary line could be an ambivert - both introverted and extroverted in different ways. So the author by saying 'it's a sliding scale' is saying introvert/extrovert aren't fixed values - they can vary. People can occupy a range of values. Two may be introverted for example, but one is still more introverted than the other.

This is a non-standard use of "sliding scale." There is no actual "sliding scale" here, but an analogy: just as with (for example) sliding scale payment, where if I am poor you charge a little money, if I am rich you charge more money for the same service--so here, if the brain is more sensitive to a neurotransmitter, a little of it has a big effect; if the brain is less sensitive, it takes a lot to have a bit effect.

It depends from person to person. They often talk about the “autism spectrum” the same way. Some people have many introverted characteristics and some people do not.