According to my norwegian classes, si-sin-sitt are 3rd person posesive reflexives (like hans, hennes) but used when the subject is in the sentence ("Han elsker bilen sin" vs "Den er bilen hans") If hans/hennes is used while the subject is present in the sentence, then the posesive refers to another person being the owner and not the subject ("Han elsker bilen hans" = John loves Peter's car)
I believe si-sin-sitt is used when the item belongs to the subject, and hans is used when the item does not. e.g: Abby and Bridget are in the park, and Abby kisses her husband. If 'her' in this sentence means Abby's, use si-sin-sitt, meaning HER OWN. But if 'her' means Bridget's, then you would use hennes. tl;dr: si-sin-sitt = his/her own
Taking it from the top...
There's two common ways to express possession in Norwegian. The possessive word can come before or after the noun.
If it comes after, the noun goes into the definite form; and if it goes before, the noun goes into the indefinite, as you know.
In terms of difference in meaning, the general rule is to treat after the noun as default. If you put the possessive word before, generally that puts more emphasis on it.
Det er bilen min, ikke sykkelen min.
That's my car, not my bicycle. (Default way of expressing it -- usually the key detail is the thing possessed, not the owner.)
Det er min bil, ikke din bil.
That is my car, not your car. (Here, the owner is the key detail.)
There may be other differences as well. For instance, putting the possessive first is commoner in writing, and can come across as more formal. And there may be various set phrases, etc.
In this case, you couldn’t use “eplet sitt”, because the apple itself is the subject of the sentence.
But if you had “He is eating his (own) apple”, you could write “Han spiser eplet sitt”.
If he’s eating some other dude’s apple, then it would be “Han spiser eplet hans”.
In general, treat possessive word second as default. This puts emphasis on the thing possessed. “Bilen hans, ikke huset hans”.
If you want to put emphasis on the possessor, you can put it first: “hans bil, ikke hennes bil”.
It’s more complicated than that (for instance, possessive first is commoner in writing, without necessarily any special emphasis). But that’s the normal pattern.