One scenario could be when other students in Hogwarts try to bully Harry. Fred and George might use their pranks to divert the bullies' attention or make them look foolish, thus protecting Harry. For example, if someone tries to hex Harry in a corridor, they could quickly set off a dungbomb near the attacker, causing chaos and allowing Harry to escape.
They could also be protective in a more subtle way. When Harry is facing a lot of pressure from the media or the wizarding community, Fred and George might spread positive rumors or funny stories about Harry to lighten the mood and make people see him in a more favorable light. Maybe they start a rumor that Harry has a secret plan to make Hogwarts even more amazing, which takes the focus off any negative attention on him.
They might use their humor. For example, if someone is making Hermione feel bad by teasing her about her studious nature, Fred and George will crack jokes at the teaser's expense, making Hermione laugh and feel better, which is a form of protection in a way.
One common scenario could be in a dangerous situation at Hogwarts, like when there's a dark magic threat. Cedric might step in front of Harry to shield him. Another could be during a Quidditch match where an opposing team is playing dirty, and Cedric ensures Harry is not targeted unfairly.
Another scenario might be when Percy is in a dangerous situation like facing a powerful monster. Reyna would not hesitate to put herself in harm's way to protect him. She might shield him with her own body or use her strategic mind to come up with a plan to keep Percy safe.
Often, during the Triwizard Tournament, Viktor will keep a close eye on Hermione. If there are any tasks that seem overly dangerous or if other competitors try to play dirty around Hermione, he'll step in. He might offer her advice on how to deal with the challenges based on his own experiences in the tournament. He's not just physically protective but also looks out for her emotional well - being. If she's stressed or worried about the tournament, he'll be there to comfort her.
Friendship. Harry is their friend, and they care about him. Just like any good friends would, they want to keep him safe from harm.
Maybe in a social setting where there are rumors or false accusations against Harry. Blaise takes it upon himself to investigate and clear Harry's name, showing his protectiveness in a more subtle but important way.
In one scenario, when Percy is about to face a powerful enemy alone, Reyna steps in and stands beside him, ready to fight together. This shows her protectiveness as she doesn't want Percy to take on the danger all by himself.
Often, in such fanfiction, Itachi protects Naruto from the Akatsuki. Since Naruto is the jinchuriki of the Nine - Tails, the Akatsuki might target him. Itachi, being part of the Akatsuki but having his own motives, would ensure Naruto's safety from his comrades. He could mislead them about Naruto's whereabouts or lie about Naruto's power level to keep them from making a hasty move against him. Itachi might also protect Naruto from the villagers' negative attitudes. Naruto was often ostracized in the village, and Itachi could use his influence or just his presence to make the villagers think twice before treating Naruto badly.
Fred often uses his humor to diffuse tense situations that Hermione might be in. For example, if she's being harassed by some bullies, he'll make a joke that makes the bullies look stupid and also makes Hermione feel better.
You can start by looking on Fanfiction.net. It has a large collection of fanfictions, and you're likely to find quite a few about Harry, Fred and George there. Just use the search bar and type in their names.