It starts all innocent with some people trading information on the incident with the Chamber of Secrets in the summer of 1993. With so little information available, many start to throw in theories and postulate on probabilities. When an older sibling hears the theories and is struck by inspiration, the first "friend-fic" is born. It quickly is spread by duplication charm and word of mouth. The final reveal that Voldemort was a time-traveler captured the minds of many. By the time Harry is on the train back to Hogwarts, "friend-fics" are pervasive and the "in" thing to discuss.
Meanwhile others have begun to use it to explore different interests and topics in a safe manner. "Ron" disliking being forced to capitulate "Hermione"'s desire for snogging on her schedule. "Harry" being forced to enjoy "quidditch" by "Ron". A slew of stories end up centered around having Harry, Ron, and Hermione be caricatures for short vignettes about controversial topics. Often these would be recommended among friends as to not rock the boat but bring attention to topics. While there is a bit of squick territory in the earliest of these fics, authors quickly pick up the technique of aging up the trio to appropriate ages.
Then the Hufflepuffs strike gold. At first their stories were more of the same until AbbottAddict (Susan's pseudonym) releases her debut fic, "The Boy-Who-Loved-Me". From there is an explosion of shipping fics. Many seeking to mimic AbbottAddict's style for the praise and adulation, but some seek to explore topics which they can't in real life. Many outside Hufflepuff desire to know the who the author is, but Hufflepuffs stick together (even if Susan is often pushed to pair up with Harry in Herbology).
With the new wave of shipping fics, new tropes begin to play out and it becomes simple to see where authors originate. (edited)
Gryffindor authors often have a flare for the dramatic and love a good noble protagonist often posing Harry as a princely knight saving a maiden. The romance is often pure and the reward for the protagonist. Any smut that comes from them is often circumstantial where Harry is foisted into situations where he must bed the maiden (not that either mind, but it's never Harry's own desire to).
Ravenclaws have a penchant for world building often to the detriment of their character development but their portrayals of Harry often have him hide his true talent and sneer at anything anti-intellectual. The smut that leaks from Ravenclaw is often held as the most sensual and spicy. Their expansive vocabulary and use of figurative language to illustrate sex often leaves the reader hot under the collar and on their bed with the curtains drawn.
Meanwhile, Hufflepuffs tend to fall into different camps with smut fiends having Harry be shared amongst a coven of sister-wives or those that call themselves TruePuffs having Harry save the world adventures through the 'power-of-friendship' and a Harry who will do anything for a friend. Most people don't begrudge them their wish fulfillment and escapism, but it is widely regarded as subpar.
Finally there is the mystery of Slytherins. The condescension towards other houses and their love of rigidly following Chekov's Wand easily gives them away, but they have surprising variety in their portrayals of Harry. A Harry who defies the "Light" for a pure, proper pureblooded heiresses because he can't control his love for her. A Harry who steps in and protects Slytherin and demonstrates that they are not all evil Death Eaters. A Harry who is forced to marry a downtrodden pureblood due to a long standing contract that became realized when he turned of age. A Harry who takes who he wants whenever he wants. Many male Slytherins are more upset by the amount of fics centered around them entering a tryst with Harry than the fact that Harry ends up stealing all their women in most fics. Even with the Slytherins being the most vocal of their dislike of being partnered with Harry, there always seems to see a lot more green when a new update is available for Harry/Slytherin fics.
For many Harry's social position and relative distance allows for authors to use him as a tabula rasa, an empty canvas, to fill with their own picture of Harry. Of course this leads to many relationships to enter rough waters when either partner happens to make an appearance. After all, where there is smoke there is fire right?
By the time Halloween rolls around, Harry finds out about the fics and is baffled. Some hit a bit too close to home, others have him in stitches, and some leave him needing a cold shower. He cannot understand the fascination that people have with him, but he becomes aware of a lot of little things about wizarding culture through the fics. Ultimately, he knows he has no say in this and tries to make due. The information he gleans from the stories lead to him purchasing new clothes, making new friends, and mending bridges long before they are irreparable. (edited)
Unfortunately, he finds a fic that is incomplete and hasn't been updated in a long time but he has to know the ending. So Harry enters the dark underbelly of Hogwarts "friend-fic" scene to hunt down the mysterious CandlelitWriterXoXo. And if anyone asks it's for research purposes!