I looked all through the documentation and couldn't figure out how to do this in 1.16.5.

In 1.12.2 if I wanted to add all items in oredict egg to oredict listAllegg I would do this:
ore:listAllegg.addAll(ore:egg);

I can't seem to find an equivalent for this in 1.16.5; the migration page doesn't mention it, and the pages in the Vanilla/Api/Tags section don't mention anything similar either. I tried .addAll, and that doesn't exist, and doing tag:items:forge:egg.add(tag:items:forge:eggs); also caused an error.