Diagrams uses .drawio.svg extention and rendered using simple img tag inside a div (almost like any other iamge). When the image is clicked, draw.io editor is open in the same page. Edited diagrams are saved to wiki uploaded files and the page content is updated to point to the new file url.
New diagrams can be added using a non-exiting image (e.g, ![NewDiagram.drawio.svg](NewDiagram.drawio.svg)). It will be rendered as a button to create a new diagram when clicked

Dark-mode is supported also