This is a complex project and I'm not sure what is the correct language to use, but I thought asking here would be a great start.
I operate a react-native mobile application that stores images and videos of users during an activity. At the end of their activity we would like to combine text overlays, images and videos into one video that can be shared. What I need is very similar to how facebook creates memories, friend anniversary, and year end videos. EG: this
We currently store the assets in firebase. Our developers can create an api for you to get the url of the assets. Assets are stored as .jpg and .mp4.
Thoughts on the first steps to create this?