Two entire chapters of a 'CQ-How-To' book could be devoted to the subject of 'Design(er)' in CQ. If you've been through the Adobe Developer training, re-read Day 3, 'Control the Brand - Design Dialogs'. Also, there are many documents and blog topics dedicated to Design(er) mode. For example:
dev.day.com link for Designs and the Designer:
Two part tutorial from Marcel Boucher, of Adobe, on Adobe's blogs:
Outside of that, I can extol to you the value of design mode from years of project development in which, not only is the output css/js specific to a given page, but entire designs can be changed across one site with simple css changes. Not only that, but entire sister sites can be created using the same templates, but then simply changing the logo, branding, etc. Then there are all the topics that go along with users/groups/permissions with regard to controlling access to given sections of the site based on anchoring design templates.
Hopefully some of those links have answers to the specific questions you are looking for. If not, add additional comments for clarity.