$\DeclareMathOperator\Gal{Gal}\DeclareMathOperator\Spec{Spec}$I am looking for a comprehensive reference on the theory of the Grothendieck ring of varieties over a field $K$, denoted $K_0(K)$ here, that includes answers to such basic questions as:
(1) If L/K is a Galois extension of fields, then $\Gal(L/K)$ acts on $K_0(L)$. Is "$\Spec(K_0(L))//\Gal(L/K) \simeq \Spec(K_0(L))$" as stacks?
(2) What are the prime ideals of $K_0(K)$?
(3) If $R$ is an integral domain, $K$ its field of fractions, and $k$ the reduction of $R$ modulo a prime ideal, what is the relation between $K_0(K)$ and $K_0(k)$? Is there a nice fibration?