This is a wild collection of hints how to do things when extending Pages itself, developing components etc., in no particular order. This is not about how to use Composum Pages.
Consult http://localhost:9090/system/console/clientlibs to get an overview what is there, find categories etc., and find examples where libraries are defined and declared and of js / css files.
Entry point for many dialogs: pages/stage/package/src/main/content/jcr_root/libs/composum/pages/stage/edit/js/dialogs.js
Compare ToolsCollection and classes using that: provide a collection with
resource type composum/pages/tools/collection) with children that declare that extension. Compare class
comment in ToolsCollection.