In Russian Doll, there were so many instances where we see fruits are rotten:
Not only in Beatrice's house and the store but it was everywhere including Ruth's house too.
Later Nadia even shows that fruit is still fresh from inside:
But what was the reason for it getting rot?
In this answer, I Surmised that the rotting fruit represented Alans diminishing control of his surroundings, Alan being completely insecure requiring everything to be perfect and predictable, the fruit rots more, the further things spin out of control and the less control Alan has.
Nadia also used the fruit to show the audience that time is not a linear A——>B system, but infact something else, whereby objects, or parts of objects experience time in a different way. As with all time travel/loop shows it can be complicated and not directly obvious to the viewer what’s happening
The shell of the fruit is rotten, but the core is fine.
But infact, this is also just describing our characters in general.
