I couldn't find any simple resource saying how to make Adobe Indesign tables of contents interactive, so that each item in the electronic version of the exported PDF is a live hyperlink / cross reference that jumps straight to the appropriate page.
Turns out, the reason I couldn't find any info on how to do it, or any options relating to it, or remember anything about the steps to do it is, it just happens automatically. Nice...
You don't need to do anything except make sure that, when exporting the PDF, under
Hyperlinks is ticked. (and of course make sure that hyperlinks, tags and interactive elements aren't removed by any PDF optimiser it gets run through after exporting).
The downside of it just happening automatically is, it's not so flexible. Each ToC item is rigidly one link, and any link-specific styling has to be applied to the paragraph style as a whole. You can't manually make part of it a link, since that would mean links within links. There also don't seem to be any equivalent options to the Appearance options in a manually applied hyperlink, other than applying a standard paragraph style.
You also have the option of turning table of contents items into PDF bookmarks on the Table Of Contents style page (again, remember to also tick to include these in the export), but you don't need to do this to make them cross link.