Jennifer Garner's "exhale" speech wasn't originally in the script; there were just a few "parent scenes". When Garner became interested in the role of Emily, she asked director Greg Berlanti for a scene that has her actually connect with Simon, so they built it again and wrote what would become essentially the message of the film.
This was the first film produced by a major Hollywood studio to feature a gay teenage protagonist.
Director Greg Berlanti revealed in an interview that there were clips of the "friend group" (Simon, Leah, Abby, and Nick) that were filmed without the actors' knowledge. In order to capture the authenticity of a group of friends beyond the confines of acting, the director would sometimes tell the actors to "cut" but have the cameras keep rolling. Many of these unscripted interactions of friends just being friends (including the dancing in the car scene) made the final cut.
Although the movie is titled "Love, Simon," it is actually an adaptation of the book "Simon vs. the Homo Sapiens Agenda", by Becky Albertalli. The movie originally had the same name as the book, but it was too long to say, hence the name change.