The best place to start learning is the elf audio youtube channel.
Below are the official tutorials from elf audio:
How to resample in koala
How to use mic fx
How to record from any app into koala using screen recording (iOS)
You can also do this with Android but each android manufacturer has a different way of screen recording, so find a tutorial of how to do screen recording on your particular device, then go to the menu and choose “import video” to import the video into koala.