Storing your uploaded assets in Digital Ocean Spaces is a great alternative to S3, and less of a pain in the ass than storing them in a local folder, no more having to manually sync files back and fourth.
Cache busting is telling the browser to refetch assets because they have changed, caching assets allows the browser to retain a file "in memory" without having to download it each time it visits your website, making the page load faster, however if you have done some changes to the styling or logic in your JS, you want to "bust the cache" so the browser gets the new updated files.
In Craft CMS 3.1 the long awaited project config feature was added, which, when an administrative setting (channel is added, field is created or updated, assigned to a section, or plugin installed) is updated, the project.yaml file in the config folder will store the change, you can then "sync" those changes to the production database when deploying your changes, here is how our team utilize this feature.
Learn how to mark your navigation links with an active class in Craft CMS using Twig macros and various other methods.
I've recently experimented with the awesome plugin "Blitz", which is a full page caching plugin for Craft CMS that generates static HTML files of your pages, so you can serve them ultra-fast using Nginx or Apache.