He might use Stiles' knowledge of Scott. Stiles knows Scott better than almost anyone, so Derek can learn about Scott's likes, dislikes, and habits from Stiles. For instance, if Derek wants to give Scott a gift to win him over, he can ask Stiles what Scott would like.
In some fanfiction plots, Derek uses Stiles' position in Scott's life. Stiles is always by Scott's side, so Derek can use his own relationship with Stiles to be around Scott more often. For example, he can tag along with Stiles when he goes to meet Scott, gradually becoming a more regular presence in Scott's life.
One possible scenario could be that Derek wants to gain Scott's trust. Since Stiles is Scott's best friend, Derek might think that by getting closer to Stiles, he can have an in - road to Scott. For example, he could ask Stiles to pass on messages to Scott or arrange meetings through Stiles.
It becomes strained. Trust is broken, and Stiles may start to distance himself from Scott. He might not believe in Scott's words or actions anymore, and there will be a lot of tension between them.
Stiles might initially be shocked. He has been a part of the pack for a long time, and Scott kicking him out would be completely unexpected. He could stand there in disbelief, unable to process what has just happened.
Well, in some fanfictions, Stiles might try to hide the injury at first, not wanting Derek to feel guilty. He could use his own knowledge of first aid to patch himself up as best as he can.
It could be that the author of the fanfiction wants to create some conflict. Stiles might hate Scott because Scott has something that Stiles envies, like popularity or a special ability that seems to overshadow Stiles. So, the author uses this emotion to drive the plot and make the story more interesting.
It can create a lot of drama. It might lead to Stiles feeling isolated and having to deal with the pregnancy on his own, which can be a major plot point. Maybe Stiles will then try to prove himself independent or try to find other support systems.
The story could take a dark turn for Stiles. Without Scott, he might become more vulnerable and attract the wrong kind of attention. He could end up in a dangerous situation that forces him to grow and become stronger on his own.
On an emotional level, it can be a sign of trust. Stiles wearing Derek's clothes shows that he trusts Derek enough to be that close to him, and Derek might feel honored. This mutual trust can lead to more open communication between them. For instance, they might start sharing their deepest secrets and fears, which they hadn't done before. It's like the act of wearing the clothes is a catalyst for a more profound and meaningful relationship.
In the world of this fanfiction, 'Stiles is done with Scott' might imply that Stiles has outgrown his connection with Scott in the narrative. It could be because the writer wants to explore new story arcs for Stiles that don't involve Scott. Maybe Stiles has new goals or relationships that are more interesting to the writer, so he's finished with the part of the story that focuses on Scott.
At first, it might be a tentative alliance. Stiles could be initially drawn to Peter for his knowledge. Peter, seeing Stiles' potential, starts to include him more in his plans. As time goes on, they might develop a mutual understanding. Stiles begins to trust Peter more and more, and Peter starts to value Stiles not just for his help but also as a friend or something more.