So we know what the spell says it does, generally, you make an illusion to make yourself look different wothin set parameters, and there's a check a creature can make to recognize its an illusion.

My question is, does that allow that creature to literally see through the illusion to what you look like, or do they just recognize that you _are_ wearing a magic disguise to hide your true form?

Could i say, for example, dress up as a scooby doo monster, and still retain that appearance for the purpose of not letting people know what i look like underneath the magic disguise so long as concentration is maintained? Or does passing a check mean they see past it and can pick up details about my true form? If the former, i think its a neat gimmick if you were trying to play a caster character trying to maintain an alter ego to the public.

Good question. Although it doesn’t say we can look at other illusion spells like Silent Image to infer that once discerned the creature can see through it. But it’s really a DM’s call on how they handle it

In my oppinion, if you succeed on Intelligence (Investigation) check, you will know the creature is disguised, but you won't be able to see through it to know its true form.

RAW (emphasis mine):

To discern that you are disguised, a creature can use its action to inspect your appearance and must succeed on an Intelligence (Investigation) check against your spell save DC.

Major Image or Silent Image (as ThriKreenWarrior said) has different wording addind this sentence:

[...] If a creature discerns the illusion for what it is, the creature can see through the image [...]