How does Captain America return the Reality Stone when it was inside Jane Foster when it was retrieved by Thor and Rocket?

We don't know what Cap does in returning any of the Stones that were taken during the Time Heist as it is never shown. However, there are two main possibilities of what happened with the Reality Stone.

The whole plan with returning the Infinity Stones is to return them to the exact moment they were taken as to erase the alternate timelines that were created. This is mentioned by Smart Hulk to the Ancient One in the initial conversation.

Ancient One: For each stone you remove, you’ll create a new, vulnerable timeline. Millions will suffer. Now tell me, Doctor. Can your science prevent all that?

Smart Hulk: No. But it can erase it. Because once we’re done with the stones, we can return each one to its own timeline. At the moment it was taken. So chronologically, in that reality, the stone never left.

_{Avengers: Endgame}

It is later repeated by Smart Hulk when he's talking to Cap about sending him on the return journey.

Smart Hulk: Remember, you’ll have to return the stones to the exact moment they were taken. Otherwise, we’re still looking at a whole bunch of nasty alternate realities.

Steve: Got it. I’ll clip all the branches.

_{Avengers: Endgame}

Potentially all Steve does is take the Reality Stone back to Asgard at the moment it was taken and leave it under their care. He doesn't need to put it back into Jane and I doubt they would even let him.

However, he likely does have access to the device Rocket used to extract the Aether from Jane in the first place. That means he could sneak in, poke her with it and then get lickety-split.

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There are two problems with this second theory though; one, the Reality Stone is in stone form when taken back so it might not be able to go back to sludge form and two, I don't spot the extractor in the briefcase when he goes back and it is unlikely to be on his person.

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