(at around 1h 13 mins) When Jay & Silent Bob are talking to Ben Affleck and he (Affleck) introduces his daughter. The little girl is actually Jason Mewes' real-life daughter, Logan Mewes.
Kevin Smith claimed that Stan Lee was originally meant to play a pivotal role in the film's third act. Unfortunately, Lee sadly passed away before any of his scenes could be filmed. After some rewrites, Smith revealed that the film would contain a "tribute" in Lee's memory.
This film was announced on Kevin Smith's Facebook page when he made clear that Clerks 3 and Mallrats 2 wouldn't be made. Two weeks before Jay and Silent Bob Reboot was released, however, Smith revealed that after a day of signing movie memorabilia with Jeff Anderson and Jason Mewes, they had so much story material that they decided that Clerks III (2022) was definitely back in production again.
(at around 36 mins) In Milly's room you can see a poster for a movie called "Bussing" with multi colored letters on a white background. This was Kevin Smith's first idea for a movie about busboys, which grew into Clerks (1994), his first film. Hence why the poster is in the same style as the Clerks poster.
This is the first collaboration between Kevin Smith and Ben Affleck since Clerks II (2006). They had a falling out which lasted several years, with Smith confessing that he "has a big mouth and told too many candid stories that sometimes weren't his to tell". In early 2019, Affleck was interviewed for Triple Frontier (2019) where he indicated that he had not been contacted for this film, but would be available. Smith subsequently reached out to Affleck, and stated that their reunion was "emotional" and "brought back a massive missing piece of my heart". He wrote Affleck's part (a reprise of his Holden McNeill role from Chasing Amy (1997)) into the movie one week before the end of production, with the scene shot on the very last day. Smith called the scene "one of the best bits of cinema I've ever been involved with: it's absolutely magical and life-affirming and all the things I really care about now, post-heart attack."
Kevin Smith: Originally, the character of "Kevin Smith" was never to appear in the movie, with Stan Lee serving a much bigger role in the film's third act. However, after the passing of Stan Lee, Kevin Smith added himself to try and fill the void.