This was something I wanted to figure out recently, as I don't want to be relying on (dodgy?) third-party sites to grab APKs when researching.
Here is how you fetch an APK off your device using ADB. You'll need to have ADB already setup on your machine and connected to your device.
Execute the following, replacing the package name for the one you want:
adb shell pm path nl.efteling.android
You should get something like this:
This is the location of the APK on your phone! You can now use adb pull to grab it.
Replace the path with the one your previous command returned above:
adb pull /data/app/nl.efteling.android-ROYXqJqIxXW3twOBvm2JoQ==/base.apk