He might also use his wits and knowledge of the wizarding world. For example, he could use a vanishing cabinet or a secret passageway that Bellatrix doesn't know about. Since he has spent a lot of time at Hogwarts, he may be aware of some hidden routes that could lead him to safety.
Well, Harry might discover some hidden passageways in Voldemort's lair. Since he has a lot of experience with tricky situations at Hogwarts, he could use his wits to find a way out. Also, he could try to appeal to any shred of humanity left in Voldemort's followers, if there are any, and get them to help him escape.
In some fanfictions, Bellatrix might kidnap Harry with the intention of turning him to the dark side. She could try to corrupt him by showing him the power and 'glory' of the dark magic she practices. This would be a huge psychological battle for Harry as he has always been firmly against the dark arts.
Harry would likely be defiant. He has faced Voldemort (Tom Riddle) before and he's not one to cower easily. So, he'd probably start looking for ways to escape right away, using his wits and any small magical items he might have on him.
He could develop a more cunning side. Bellatrix is a sly character, and Harry would need to learn to be crafty to survive in her world. This would be a big change from his usually straightforward and brave self. For example, he might learn how to deceive Death Eaters or other threats.
At first, their relationship would be full of distrust. Harry would be wary of Bellatrix as she has been an enemy for so long. But as the training progresses, he might start to see a different side of her. Bellatrix, on the other hand, might develop a strange sense of pride in Harry's progress.
Harry would probably be very defiant at first. He has faced Voldemort (Tom Riddle) before and he won't go down without a fight. He would likely try to use his magic skills to escape or at least hold Tom Riddle off until he can find a way out.
Ron would be furious. He would immediately start looking for Ginny and would probably want to confront Harry without thinking. Hermione, on the other hand, would be more rational. She would try to figure out why Harry would do such a thing and look for clues. The Weasley parents would be extremely worried and would likely reach out to all their contacts in the wizarding world for help.
It would be a very strange and complex fanfiction. Bellatrix is a Death Eater and much older than Harry. But in such a fic, perhaps it could start with some sort of dark magic influencing Harry's mind, making him see Bellatrix in a different light. Maybe he would be on a mission to understand the Death Eaters better and in the process, he gets too close to Bellatrix and they end up having a relationship that defies all logic and the norms of the wizarding world.
Well, it would be very strange. Bellatrix is a Death Eater and Harry is her enemy in the original story. In this kind of fanfiction, perhaps Bellatrix would keep Harry captive in some way and treat him like a pet, which is quite a dark and twisted concept.
In such a fanfiction, Harry's life would be extremely different. Bellatrix is a Death Eater, so Harry might be indoctrinated into dark magic. However, his strong will from his original upbringing might resist this. He could be in a constant internal struggle between the light he was initially inclined towards and the dark teachings of Bellatrix.