Things that I can confirm don't work -
1. Adding an author box below post for EAT
I imagine Google judges authority depending on things it can measure objectively - brand mentions on other websites, and links from other authoritative websites in the same niche.
There is lots of advice about adding author information on blog posts, but Google can't be sophisticated enough to understand whether the author is qualified (to be an expert). That is subjective.
For example, what makes a good garden chair expert?
In YMYL niches, its just as cloudy. If you had a medical problem, would you see a nurse, a pharmacist or a doctor? Obviously, it depends on the problem but also the experience of the expert. Is an article written by a nurse with 20 years of experience in general practice (including the problem area) more or less authoritative than an article written by a newly qualified doctor?
Even if Google is sophisticated enough to work out relative expertise in a subject area, we need to consider whether it has the processing power to apply that to billions of pages of content.
If there is a simpler way of being nearly as accurate, Google will take it.
Google has always rated links. For me, it makes sense that if other authorities in that niche are linking to you or mentioning you, you're going to be seen as authoritative.
We know that Google is quite good at determining what a webpage is about (or at least, whether two pages are about the same subject).
I think Google has tweaked the algorithm so that subject relevant pages on websites that have a lot of similar subject material transfer more juice (or rather, subject irrelevant pages on general sites pass less juice than they used to).
That means that if you want to rank better for food supplements, you need links from blogs about that supplement written by health experts, not any old general blog that happens to link to you. It doesn't matter whether your links were acquired in a whitehat or blackhat way. If they aren't on niche relevant pages on same topic websites, you don't get as much credit for them.
Where author bios do come in is if other topically relevant sites link to your website, and your website links to the articles you've written (because you publicise what you have written on other websites). Say you're a SEO expert writing as a guest for Moz. Hopefully, you'd have lots of SEO related sites linking to your personal website quoting you as an expert. Your personal website would have high authority. Linking that to your Moz article would transfer juice to Moz, and the Moz article would rank better. Its not the author bio in the Moz article that improves the rank, its the fact that if you're promoting yourself as an expert on another website, you're going to link to your article from your own niche relevant website.
Obviously, this is all speculation - it would be interesting to see if anyone's experiences match this theory.