I'd suggest starting at Chapter 95. It smoothly picks up where Season 3 leaves off and continues the exciting plot. There are new characters and challenges introduced that will keep you engaged.
You can start from the chapter that corresponds to where season 2 ended. Usually, you can find this information on manga-related forums or from fellow fans.
The best places to read the Demon Slayer manga after the anime are dedicated manga apps like Manga Rock or Manga Plus. These offer a wide range of chapters and often have good reading experiences. You could also look for digital versions on the publisher's website.
The end-of-the-movie easter egg referred to the movie clips that appeared after the end of the movie, when the credits scrolled, or after. It was usually a humorous scene or a plot clue related to a sequel. There were several purposes for the end-of-the-film Easter egg. First, it was to let the audience show respect to the actors and staff in the play and enjoy the ending song. Second, as a series of movies, the end-of-the-film Easter egg sometimes played the role of the next trailer. Third, from the perspective of narratology, if the end-of-the-film stopped abruptly, it would give the audience a sense of unfinished. The release of the Easter egg would have the effect of a "small climax", achieving the realm of "words are finished but the meaning is endless" and the beauty of lingering sound. Therefore, if the movie had an Easter egg after the end credits, the audience could expect a humorous scene or a plot clue related to the sequels after the credits scrolled.
Well, in some horror movies or horror - themed shows, there might be. For example, in certain slasher flicks, they put a short scene after the credits that either sets up a possible sequel or just gives one last scare. It's all about building that horror atmosphere and keeping the audience on their toes even after they think the story is over.