These are admonitions. GitHub is playing with a concept right now, in beta, but it isn't fully implemented and the format may change from what is shown in the discussion and what you can do on the platform at the moment. Admonitions are done differently across tools. Dendron has an open feature request on this, if you'd to give it a thumbs up emoji in the issue or contribute to the convo if there are extra thoughts you may have around it: https://github.com/dendronhq/dendron/issues/1287