There are many ways it could be written in fanfiction. Ginny might use her knowledge of the wizarding world and Hogwarts' secrets. She could use a special spell that she learned from an old book in the Hogwarts library. This spell could create a portal right inside the Dursleys' living room that only Harry can see. Ginny could then communicate with Harry through a magical device, tell him about the portal, and he can use it to escape from the Dursleys. Ginny would have to be very careful and plan everything precisely to avoid detection by the Ministry of Magic, as using magic around Muggles without permission is strictly prohibited.
Well, it could be that Draco has a change of heart early on. He realizes that the Dursleys are truly awful to Harry. So he uses magic to create distractions. He could make strange things happen around the house that scare the Dursleys so much that they just want Harry to leave. Then Draco could be waiting outside and whisk Harry away to a safer place, like a hidden place in the wizarding world where he has set up a little haven for Harry.
Well, in some fanfictions, Bill might use his magical skills and knowledge. He could cast spells to create distractions for the Dursleys, allowing Harry to escape unnoticed. For example, he might make a flock of magical birds fly into the Dursley's yard, causing chaos while he smuggles Harry out.
Well, it could be that Harry discovers Draco's plan in advance. He then sets up a counter - plan. When Draco tries to harm Ginny, Harry jumps in, using his invisibility cloak to his advantage, surprising Draco and rescuing Ginny.
In some fan fictions, Voldemort might save Harry from the Dursleys out of a newfound sense of self - interest. Maybe he realizes that Harry has some power or knowledge that he needs, and by rescuing him from the Dursleys, he can gain Harry's trust and access that. For example, if there is a prophecy that only Harry can unlock and Voldemort believes it will help him achieve ultimate power, he would take Harry away from the Dursleys who are mistreating him.
Ginny would likely be protective. She loves Harry and knows about his difficult past with the Dursleys. So she would probably stand up for him right away, not tolerating any of their mean remarks.
It could be out of loyalty to the wizarding world. Harry is seen as an important figure in the fight against the dark forces. Fleur realizes that Harry's safety is crucial for the future of the wizarding community, so she saves him from the Dursleys who are a threat to his well - being in a different way compared to the dark wizards.
Perhaps Snape uses magic to create a distraction at the Dursleys' place. He could make strange noises or apparitions that scare the Dursleys, and while they are in a panic, he grabs Harry and disapparates. This would be a quick and efficient way for him to rescue Harry.
Maybe Percy uses his magic skills to create a distraction. He could conjure up some strange creatures outside the Dursleys' house. The Dursleys, being afraid of all things magical, would be distracted long enough for Percy to sneak in and get Harry out.
Love for Lily. Since Harry is Lily's son, Snape might feel an obligation to protect him. He couldn't save Lily, but he can try to keep her son safe.
They might interact with a mix of caution and boldness. Harry, out of habit, would be cautious not to upset the Dursleys too much. Ginny, though, could be bold. For example, she might directly ask Vernon why he's so against magic when it can do so many wonderful things. Harry would be in the middle, trying to translate the Muggle - magic misunderstandings between them.