The source code for Custom Maps is available at GitHub (
https://github.com/markoteittinen/custom-maps). That github link is also included in the app description on the Play Store listing in the "About this app" section, though Google nowadays hides most of it from the "front page". But the github link is also included in the "About Custom Maps" page (which is also displayed when the app is launched the first time).
So you didn't look very hard, and that makes me include the following comment: Use the source code to study and learn programming. Please do not "steal" it to use it as your school project or some such.