Resolution of an example video: 640x788
Desired resolution of the new video: 1920x1080
The aspect ratio of the video should be kept (the area left & right should be filled black).
My current call looks like this: ffmpeg -i input.mp4 -s 1920x1080 -r 60 -aspect 1:1 -strict experimental output.mp4. The problem here is that the video is sometimes made narrower / wider (aspect ratio is not kept).
Since I am no FFmpeg expert I hope for help here (maybe there is a padding property?)
You would use the scale and pad filters to accomplish this.
ffmpeg -i input.mp4 -vf scale=1920:1080:force_original_aspect_ratio=decrease,pad=1920:1080:-1:-1,setsar=1 -r 60 output.mp4
Use ffmpeg v4.1 or newer for this - the -strict option hasn't been needed for typical MP4 output since 2015 so your ffmpeg build may be very old.