If you look for the ‘Collection List’ in Purple in the navigator, then ‘settings’ you can see where an on page element/list is connected to an actual ‘Collection’.
You can make use of those collections (Categories and Projects are both ‘Collections’) as is, add new fields to them, or delete fields. You can also delete them totally and make your own from scratch.
If you like the current style and layout, but those two collections won’t work for you, you will need to unlink them in order to use your own (look for the purple elements in the navigator - go to settings and ‘unlink’ them). Then link them to your new custom made collections (categories or whatever you will need for your site).
Have a look at these tutorials to get a good grasp for how Collections work (the CMS) and Dynamic lists (how you show them on the page) plus watch the full site build to see an example of the process:
Full site build:
Hope those are useful