The writeup for Orcus has tooltips for The Dungeon Master's Guide and for madness. However, I can't find tooltip tags to create such tooltips. How does that work?

If a creature goes mad in Orcus’s lair or within line of sight of the demon lord, roll on the Madness of Orcus table to determine the nature of the madness, which is a character flaw that lasts until cured. See the Dungeon Master’s Guide for more on madness.

I believe those are just strict hyperlinks.  Tool tips create a little pop up reminder of said tool tip but also allow for link to the source.

As hollowptm said, those are just links. Every type of tooltip and how to use it is listed here: https://www.dndbeyond.com/forums/dungeons-dragons-discussion/homebrew-house-rules/9811-how-to-add-tooltips