To pull from (download) or push to (commit) the GitHub repo "of record", use the "Sync" pop-up in the "..." menu in the upper right of an open hackmd.
Whimsical can be a bit finicky in how it is currently paywalled— you are limited to very few documents you can store and update. Once you've created an account and logged in, you can use the "copy to" pulldown and "fork" a local copy of a diagram and edit to your heart's content.
This will create a new URL and can seen by anyone as per the sharing options, which you find by clicking the "send" (paper airplane icon) options and clicking "get shareable link," which pops open this dialogue with the permissioning options:
