You know that feeling you get when you leave a movie that you know you'll love forever? You might be asking yourself "why isn't every movie like tht one" or "is that really the only place I can see characters like that", and you won't be the only one. Fans are people tht always want more, and while Marvel is hustling to deliver as much as possible, they're still a limited resource. In order to deliver the same quality work, they need time. Which is where fans step in.
We Are, a fanvid created by Youtuber Shep is a good example of a gen trailer ("gen" as in "general", or not attached to any pairing or individual character) for the first Avengers movie. Fanvids like this often feel like extended or additional trailers for the films, incorporating some of the more exciting shots, without revealing too much of the plot. Fans obviously create fanworks for different, individual reasons. Some are trying out a new skill editing or syncing content. Other fans are using the videos to celebrate the thing that they love.
This trailer made by Elinor X makes a compelling case for a solo Black Widow origins movie. The vidder used footage from the MCU as well as films that Jeremy Renner and Scarlett Johansson have appeared in, and arranged the clips to tell an engaging story. Since a solo Black Widow movie was nowhere to be seen on the MCU's current planned timeline, fans often rely on one another to create the kind of content that they want to see.
This vid by stevesbuck is in another genre entirely: the crack vid. Videos like this often draw from multiple sources to create funny, often flippant mashups that poke fun at the characters and the fans. This one, in addition to pulling clips from the MCU, uses a scene from The Office to demonsrate the vidder's emotional distress regarding a particular scene. Often fans end up speaking an insular language, which can sometimes be confusing or alienating for outsiders. Fanvids are one of the ways that fans communicate with each other, actively expressing their passionate feelings about the media they consume and enjoy. Viewing fanvids can offer insights into what fans are thinking.