Apk extractor from F-Droid doesn’t seem to work. App in question is Typewise Offline (android keyboard with hex-shaped keys).
EDIT: Probably worth a separate post at some point, but this is one of the few apps for which there is no open source equivalent, TTBOMK, and one of the few remaining proprietary apps that I rely on.
I thought an APK file was basically a ZIP file with a standardized structure. You should just be able to rename the extension to ZIP to open it up and view the contents.
Not like that’s going to decompile the code and all, but that’s the first thing I’d start with.