The QWeb parser looks for special attributes (directives) in the templates and replaces them with dynamically generated HTML.
When displaying a kanban view, the internal steps are roughly as follows:
read()method to get the data for the fields in the templates.
kanban-boxtemplate and parse it using QWeb to output the final HTML fragments.
This is not meant to be technically exact. It is just a mind map that can be useful to understand ...