I'm a noob here, and it seems like everyone 'just does it' with no issues, but I am lost as to how to get the tooltip linking to source material on the Campaign Notes without hacking the HTML.

I found this post: https://www.dndbeyond.com/forums/d-d-beyond-general/d-d-beyond-feedback/7901-allow-us-to-link-homebrew-creatures?comment=10 (Thanks @Damian_the_DM) but this seems like a guide to doing it for the homebrew stuff, and people just happily link to the compendium stuff directly.  I made a link to Bag of Holding but it just shows up as a link, not the fully tooltipped link.

And I can't for the life of me work out how to easily add stuff. I don't want to have to hack the HTML every time I add an item.  What am I missing?  

Those “links” are called “tooltips,” and you can learn all about them here: (https://www.dndbeyond.com/forums/dungeons-dragons-discussion/homebrew-house-rules/9811-how-to-add-tooltips). That’s how you can make this: “fireball,” into this: “fireball.”

Oh great! So Bag of Holding