So I have wondered what happenes of you delete the DJI fly app and then reinstall it. Will all the videos and photos stored in it be deleted?

Yes, it will clear the cache files in the app. The files aren't stored on the app though they should also be in your device storage if you transferred them.

Not sure what OS you are on.
But usually the cache is deleted when an app is deleted.
The files should still be on the drone's sd card or internal memory. (depending on drone type)
For MM there is no internal memory, so the sd card is where they should be.

If you have an iPhone you can have the videos/photos saved to your camera roll.

Allow iOS photos to link with the fly app and enable the camera to save to photos (thats if you are referring to an iPhone

At least on MY Android setup the app also stores low resolution files in
DJI/dji.go.v5/DJI FLY/Video.