title: What am I seeing when I build an angular library (bundle, esm5, esm2015, fesm5 and fesm2015)
body: <p>Trying to understand what is output from build an Angular8 library (since it builds 5 similar looking things, the build time has gone up for each library) - What am I seeing when I build an angular library (bundle, esm5, esm2015, fesm5 and fesm2015)? Can I teach the Angular8 to only give me ESM5 or UMD modules for my libraries and not spend extra time generating these unwanted "modern browser" stuff? </p>
  
  <p>I read a bit about differential loading but does not seem to make sense from the point of view of building Angular8 libraries.</p>
  
  <p>Any pointers are appreciated.</p>