Do "sleep in" and "oversleep" mean the same thing? If so, what is hidden after "sleep in?"
Assuming "sleep in" and "oversleep" mean the same thing, it seems there's something missing in "sleep in." What is supposed to follow after "sleep in?"
Although the use of in may imply being in something, it's not said.
It has much the same usage as stay in:
"Are you going to see the carnival?"
"No, I've decided to stay in."
As commented, it usually indicates a deliberate (or at least, non-accidental) action. That nuance may not be entirely clear in dictionary definitions.
sleep in Phrasal verb
Remain asleep or in bed later than usual in the morning.
‘life assumes a different rhythm on the weekend; we sleep in, cut the grass, wash the car’Sleep by night at one's place of work.
— Lexico
Oversleeping is generally not intended to happen.
Sleep longer or later than one intended.
‘we talked until the early hours and consequently I overslept’— ibid.
Sleep in often means intentionally staying longer in bed
It doesn't necessarily mean asleep at the end interestingly enough, and might include activities such as breakfast in bed
He knew that he would have a good sleep in the next day
Oversleep often means accidentally sleeping too long
Suddenly she woke up and realized she had over-slept
If you are late for work you would apologize for having over-slept but if you said you had a sleep in your boss would give you a strange look (US culture).
Language Log gives an interesting insight on the use of the preposition “in” for the expression “sleeping in” meaning sleeping late:
The original idea of the "sleeping late" sense is that "in", meaning "in one's house" or "in one's bed", is opposed to "out", meaning "out in the world" or at least "out of bed". In addition to the OED's reference "to lie in", there's the verbal form "to stay in", and the noun "shut-in", with similar meanings of "in one's dwelling". The idea of being "in" rather than "out", associated with sleep, then takes on the extra connotation that one is staying "in" while sleeping during a period of time when one normally would be "out".