>  “you are saying I am doing X bad thing? What about Y bad thing you’re doing, let’s discuss that instead”.

no one is generally expecting or even asking for "let’s discuss that instead". what they are doing is trying to point out that your lack of of care about Y implies that you never actually cared about X in the first place but rather you only care about me doing X.

often times X is actually bad but human nature is such that no one actually cares about it while wanting to appear to when locked down on it. for this reason arguing about X directly is bad optics.

whataboustism is basically an effort to draw attention to the selective enforcement of norms, laws, morality, whatever. hypocrisy is implied.