How and when are the esm5 and esm2015 directories of a "ng build"-generated lib folder used?

While updating my angular version, I think I may have found the answer. The library that will be used is dependent on the browser requesting the application - The esm5 build will be used for older browsers that require more polyfills and the esm2015 libraries will be used for newer browsers that require fewer polyfills

"The CLI's build command now automatically creates a modern ES2015 build with minimal polyfills and a compatible ES5 build for older browsers, and loads the appropriate file based on the browser. You may opt-out of this change by setting your target back to es5 in your tsconfig.json. Learn more on angular.io."

From https://update.angular.io/#7.1:9.0