Shinso Hitoshi's first appearance was in chapter 23 of the manga where he stands up to Bakugo Katsuki. Katsuki rudely dismisses his classmates which Hitoshi challenges that the 1-A students will have to be careful as people like him will want to steal their spots. Izuku is more distracted by the points he brings up that Hitoshi undermining Katsuki, but takes what Hitoshi says seriously.
During the entirety of the sports fest, the minimal interactions that the two have are with Hitoshi, seemingly looming over Izuku.
Later the two have a one on one match for the sports festival finals. Hitoshi seems to have an understanding of Izuku's values as a compassionate and loyal friend to his classmates, having to bait him into answering his question by getting under his skin. He manages to get him to respond by insulting on of Izuku's classmates. Izuku frees himself, and as he begins his counterattack, Hitoshi continues berating him to get him to respond. Even with the two on opposing sides, and Hitoshi berating Izuku, he empathizes with him, as the two both share a history of being held back by quirks, in Izuku's case, not having one while Hitoshi's made people believe him to be more like a villain. The two share a brief moment of mutual understanding after their match. All Might even shares the same empathy when he acknowledges how difficult it must have been for Izuku to hear what Hitoshi had to say since he could relate.
Hitoshi was seen teasing Izuku after their match, telling him to keep his guard up, and showing respect for winning the match. He also give him a few words of encouragement which Izuku honored by taking to heart.
There's an indirect interaction when Izuku notices him walking beside Aizawa, implying that the two have been training together. The second time Izuku sees the pair walking together, he greets Hitoshi which was met with minimal response. However, later told Hitoshi has a good friend in Izuku, he smiles to himself and agrees.
The next big interaction comes during the Joint Training Arc, where Hitoshi is introduced to classes A-1 and 1-B as a try-out for the hero course. While he claims that he "isn't there to make friends", Midoriya is shown to have a smug little grin on his face, possibly in excitement for Shinso. During the training exercise, Midoriya reawakens Black Whip and can't control it. Shinso steps in and suggests a rematch to recenter Midoriya's focus and it eventually works to dissolve the situation.
The two might not have very many interactions, but the ones they do share offer fans with an established emotional connection between the two. Both want to be heroes, and have had to fight harder than most for their success. It can be argued that Midoriya's influence during the one on one match was what influenced Aizawa into training Shinso as it's revealed that Shinso had less athletic ability than Midoriya who scored in last for the Quirk Apprehension Test. Aizawa was shown to admire Midoriya's resolve, and his ability to adapt to his ability, possibly opening him up to Shinso's potential.
It can also be argued that the two are likely to become friends in the future, as they have highly compatible personalities.
The ship has much in common with TodoDeku, with both ships sharing the same height difference, and tsundere/deredere characterization.
It became the most popular after the sports festival arc when the two had a one on one fight. The two had a one on one match in the finals of the sports festival. Izuku ended up winning, however, he found the match challenging due to his empathy for Shinso's reasoning for wanting to become a Hero.
In terms of popularity, this ship rivals ShinKami, the ship between Hitoshi and Kaminari Denki.