There is great contrast: Aizen managed to insert it without Rukia knowing. later, he stated that extraction would be fatal (but it did not actually kill Rukia).
How did Aizen insert it into Rukia? And what are the plans he did so that she will be in a dangerous situation with Ichigo?
Aizen does not insert the Hōgyoku into Rukia's body. The deed was actually done by Urahara Kisuke when he makes a deal with a powerless Rukia for a human gigai into which he places the Hōgyoku for safety and secrecy.
Rukia by coincidence happens to patrol Karakura town on the day the hollow attacks Kurosaki family, the Hōgyoku belonging to Kisuke is hidden in the gigai which he gives after she transfers her powers to Ichigo; until then she can sense reiatsu and it was because of sensing Ichigo's reiatsu that she initially entered the Kurosaki residence as a shinigami. However after using the (Kisuke provided) Hōgyoku contained gigai to stay in human world, she realizes that her shinigami powers are not recovering fast enough.
Aizen likely orchestrates the hollow attack on the Kurosaki family believing this would cause Kisuke, who until then hid in the shadows, would make a move for the safekeeping of the Hōgyoku. And rightly Kisuke makes an appearance offering Rukia a deal on the gigai to use for the stay in human world.
Also to be noted that the entire birth of Ichigo is a causality of Aizen's earlier experimentation and he mentions that he has been monitoring Ichigo for a long time
As for the extraction, Aizen merely assumes/contemplates that it would be of fatal consequence and expresses surprise when that is not the case. In general Aizen's knowledge on Hōgyoku and even that of Kisuke is not complete as they only succeed in creating an incomplete one at best both of which Aizen would later merge and use.
I just watched the episode and Aizen does indeed say that Urahara hid the Hogyoku in a soul and that by the time Aizen realised it was in Rukia, she had already escaped to the human world. So, Guest's answer was right and optimusprime619's answer is wrong.
The rest is true though, he did give Rukia the Gigai that drained her powers and he did 'manipulate' and help the crew to save Rukia because he knew Aizen was after her.
I've just rewatched the anime for like 4th time and yes, Aizen said that he realized the Hogyoku was with Rukia after she had left for Karakura town back then.
Bleach has so many holes in its plot.
Did anyone listen to Aizen's speech? The Hogyoku was inside Rukia before she even met Ichigo. In fact, according to his concept, it has been inside of her since she was a child. The question is, when the heck did Urahara met Rukia when she was a child?
