I use Sony Movie Studio Platinium 14 and DaVinci Resolve 16 to edit videos. I recently tried to edit a slow-mo clip shot on iPhone XR and found that neither of these applications could properly open and display them. These videos were encoded with H.265 or HEVC (High Efficiency Video Coding) and shot at 240fps. This high frame rate is actually what makes them slow-mo. When played back at a normal frame rate or 30fps they appear as slow motion.
After doing a few tests with Windows software I could get my hand on (Handbrake, etc), I couldn't find anything that would allow to convert them into something editable in Movie Studio or Resolve.
To my help came my old trusty Macbook Pro with iMovie video editor. I simply imported the slow-mo videos and without any editing exported them in the highest quality available on iMovie (ProRes). Just make sure you choose quality as Best (ProRes), as other options may result in poor quality. The new encoded .mov file could be imported and edited in both, Sony Movie Studio Platinium and DaVinci Resolve with no issues.
Davinci Resolve 16
Sony Movie Studio Platinium 14
Windows 10 Pro (1909)
macOS 10.15 (Catalina)