Android and DEX files handling at runtime

I'm trying to understand a few things around how dex files are handled by android. What I do know is that an APK ships a dex file, the system gets it, optimises it at install time, and stores the resulting ODEX file (optimised dex) at /data/dalvik-cache/myOdexfile and then runs the application from there. Is that really how it works? What happens when a dex file downloads and opens a new dex file from the internet? is that optimised as well? Also - the system, afaik, keeps the original APK around, right? I think that is kept at /data/app/myAPK-1.apk. Is that used in any way by the system? When? Can you somehow manually force regeneration of the odex file from the stored APK file after installation?