A footnote is defined in a paragraph that is started by a footnote marker in square brackets in column 0, no indentation allowed. The footnote reference is simply the marker in square brackets, inside text. For example:
The Org homepage[fn:1] now looks a lot better than it used to. ... [fn:1] The link is: https://orgmode.org
The following commands handle footnotes:
The footnote action command. When the cursor is on a footnote reference, jump to the definition. When it is at a definition, jump to the (first) reference. Otherwise, create a new footnote. When this command is called with a prefix argument, a menu of additional options including renumbering is offered.
Jump between definition and reference.
Chapter 2 of the manual
Sacha Chua’s tutorial