To convert your landscape 16:9 videos which are good for YouTube but not great on mobile apps like Instagram, Pinterest which need a square or portrait format, there is only 1 easy step to take in DaVinci Resolve 16.
It's a savvy online marketing technique to take one video which you created mainly for YouTube, to re-purpose it for other platforms. Unfortunately other platforms may have very different suggested requirements for their video resolutions. Apps that are mostly mobile-centric require resolutions that are either portrait or square shaped.
Well, you can still use those landscape videos for these mobile platforms by converting them in DaVinci Resolve. If didn't convert the landscape videos and uploaded them to say Instagram, about 1/3rd of the video will be cropped. So it's a good idea to convert it first in DaVinci Resolve to a mobile-friendly format then upload it. Black bars will be automatically added so that the entire video image can be seen.