Detail layout is the single page as just like a WordPress post.
It has own URL permalinks which can be changed from settings.
Clicking   on gallery toolbar, will display detail page.

‘Basic Layout’ has 5 Locations & ‘Complex Layout’ as 15 locations , where we can place the different elements. In Flexi we call elements as section like category, tags, images, etc. we show/hide on the page. There is no drag/drop options to make it more flexible to developers.

Chart is a visual reference to indicate the available location for the Flexi elements. As you can see on above images.
For example in reference to complex layout : You can place Media into location no.1 (gray color) which will make media to 100% width. Similarly if you keep media location no.2 (yellow color), media will fit half of the screen 50% and you can keep other elements to it’s right side at location no.3 (green color)