Use the {checkbox} expression to insert a new checkbox somewhere.

Tap on the checkbox to make the dialog pop up (it's a regular Rich Footnote one for that matter)

Use the "Checkbox: tick" or "Checkbox: untick" buttons inside the dialog to change the state of the checkbox.

Also works on regular links if they include the checkbox in the link name 🎉! Suggestion: add the checkbox emoji to the name of note to automatically have it when referencing the note via inline tags. Changing the state of the checkbox in inline tags won't change the parent note's name, just the reference's! Great way to make habit trackers!