From Cambridge Dictionary:
Entry "the''=> 17th usage (rule # 17 out of 19) to the usage of THE.
17. used instead of a possessive adjective such as your, her or my
He held his daughter by the arm.
I can't remember where I parked the (= my) car.
My note: 1st. example: Instead of "by his arm (the arm of his daughter)
2nd. example: self-explanatory in itself, yet if we changed it to
He can't remember where he parked the (= his) car.
But then, let´s think of a situation where there´s just one car involved, but I borrowed from him, I went to the mart, so I am the driver.
I can´t remember where I parked the car (=also his) car.
This one would fall on the rule # 1. used before nouns to refer to things or people when a listener or reader knows which particular things or people are being referred to
, especially because they have already been mentioned or because what is happening makes it clear.
I hope it helps.