Apologies if this is a dumb question, I am mostly self-taught when it comes to physics. I recently came across this video by PBS Space Time (Is Gravity an Illusion?) which talks about the Equivalence Principle, where a box falling freely in a gravitational field can be considered an inertial frame, and a stationary frame in a gravitational field is equivalent to a frame accelerating in the opposite direction of gravity.
He illustrates this by saying that an apple in free-fall is an inertial frame while the surface of the earth is not, and really it is the surface that is accelerating upwards relative to the apple.
This leads me to think about the case where an apple is thrown up from the ground with initial velocity slower than escape velocity - it rises up, slows down, stops, turns around, accelerates downward, and eventually falls back to the ground. What is the equivalent explanation for the above case when the earth is accelerating upwards? Clearly the apple's reference frame is non-inertial when it is on its way up, but it becomes an inertial frame when it starts free-falling downward. That means the acceleration somehow vanishes. But since the only acceleration in the apple's frame is the one that points upward (opposite to the gravitational field), what causes it to slow down?