HI,
I'm not too familiar with the iMovie for iOS app. From my limited use of it, it appears that the media in the project is stored in the Photos app and will remain there even if the project itself is deleted. In other words, the original media exists in the Photos app, or wherever other place you have it stored, and the iMovie project just refers to it.
Deleting and reloading the iMovie app may result in deleting the projects themselves -- not sure about that. Unlike on the Mac, there doesn't seem to be an iMovie library that retains the projects independently of the app. However, beforehand you can save each project to Files on the iPhone and that may preserve them.
Hopefully, someone with more experience on iMovie for iPhone will chime in with more information.
-- Rich