Maybe they overhear a secret conversation between teachers or magical creatures that give them clues. Or it could be that they discover a hidden room filled with evidence about the Marauders.
The main characters are obviously Fred and George from the Weasley family, and the Marauders which include James Potter, Sirius Black, Remus Lupin, and Peter Pettigrew.
In this fanfiction, Fred and George might get into all sorts of mischief with the Marauders. They could learn some new pranking tricks from James and Sirius. Maybe they'd team up to play a huge prank on Snape, just like the Marauders used to do.
They are likely to be horrified. James, for example, would probably be extremely angry that his friend had to endure such a thing. He might immediately vow to protect Sirius from any further harm. Remus, being more empathetic, would be there to comfort Sirius and offer his support in a more quiet and understanding way. Peter, though not as strong - willed as the others, would still be shocked and follow the lead of James and Remus in trying to make Sirius feel better.
The interactions might start with a lot of questions. Fred would be curious about the Marauders' time, and they would be interested in his more recent Hogwarts experiences. For example, Fred could ask about how they managed to create the Marauder's Map, and the Marauders could ask about the Weasley twins' joke shop ideas.
Hogwarts: Weasley Is Our King was a recommended novel, which included supporting actors Fred Weasley and George Weasley. This book can be read on Qidian Chinese website.
One way is by protecting Harry from the bullies. In the fanfiction world, there could be students who are jealous of Harry being in the Triwizard Tournament or just want to cause him trouble. Fred and George, being older and more experienced, can step in and stop any potential harm.
One possible way is through their body language. For instance, Fred might stand with his left foot forward more often, while George has a habit of leaning to the right when he's standing still. Harry, being observant, picks up on these little quirks.
There are indeed some fred and george and harry gay romance fanfiction pieces. Some authors are really creative in imagining how such relationships could develop within the wizarding world. But keep in mind that fanfiction is a very diverse field, and not all of it may be to your taste. You may need to sift through a few to find the ones that really resonate with you.
They might use it to buy more ingredients for their products. They could purchase things like Pygmy Puffs, or other magical creatures and items that they use in their joke products.
Fanfiction.net is also a good place to look for fred x george fanfiction. There are many different categories and tags you can use to narrow down your search. However, the quality of the fanfictions can vary, so you might need to do some browsing.