How to find app based on package name?
The easiest way, without using any specialized tools is just to visit that app's page in the Play Store (website, not app).

All apps in the Play Store have their package name as part of their URL, so you can just take your package name and prefix it with https://play.google.com/details?id= to get

https://play.google.com/store/apps/details?id=com.stackexchange.marvin

where com.stackexchange.marvin is the package name which will take you to the page for Stack Exchange Android app.