I believe you are talking about the Memories feature in Photos. There is no way for you to create them yourself, but do not dispare, there are other options.
Slideshows:
If you create an album, you can run it as a slideshow. Open the album and tap Slideshow at the top right. Once the slide show starts you can tap the Optoins button at the bottom right to change the theme, music, and speed.
Video Apps:
If that is too limited you can use the iMovie or Clips app. Using either of these you can select (or record) images and videos, add titles, sound trackes, etc. Clips is meant for short videos with a personal touch. iMovie is more movie/cinema like. The results of either can be saved back to the Photos app, or shared by other means.
Both iMovie and Clips are Apple apps, and available in the App Store.
If you donât like the apps mentioned, there are many third party apps with similar capabilites.