I'm studying large cardinals and I'm hoping to fully understand the proof that says ZFC is not able to prove the existence of inaccessibles (given ZFC is consistent, of course).
I've already fully understood and/or proven everything up to and including: if we had an inaccessible cardinal $\kappa$, then $V_\kappa$ is a (sub)model of ZFC. I also understood the main proof that takes the least inaccessible $\kappa$ and arrives at a contradiction by finding another inaccessible inside $V_\kappa$.
The only thing that's missing is the following Lemma:
Given a model $M$ of ZFC and an inaccessible $\kappa$, $\lambda$ is inaccessible in $M$ iff it is inaccessible in $V_\kappa\subset M$.
I feel like the proof of this lemma is highly technical and that I'm getting confused about the relativization of the axioms and the absoluteness of the functions and properties needed to define cardinals and inaccessibility.
I haven't found a proof anywhere. Does anyone know a good reference for this lemma?
Thanks in advance!