If you go to the website of any large company and create a bookmark, regardless of what browser you’re using, that bookmark will contain the company symbol. For example, any bookmark that you create for BMW will contain the blue and white roundel which is the symbol for BMW. How are those custom bookmarks configured? And how would they be configured for a Vaadin webapp?
IIRC it just takes the favicon of the page, which can be set using the [PageConfigurator]
(https://vaadin.com/docs/v14/flow/advanced/tutorial-bootstrap.html#pageconfigurator)