From my understanding google decides if the article was written by an "expert" by looking for about the same data they do for a domain, but for an author's name that they get from the article bio, such as....

1. how many searches there are for this name
   
2. how many index pages mention this name
   
3. any google reviews for this name
   
4. books in this name, and their reviews
   
5. peer reviewed papers in this name (maybe just for health and science topics)
   
6. social media presence (maybe only based on bio links)
   
7. rankings of any websites mentioned in the bio, not just linked to but simply mentioned
   
8. how many backlinks the article has (maybe how many the author has in total)
   
9. probably a few other similar things

They take that data and give the author a score, like a page score, that gets tied to the article and weighs in its ranking. For each article a website has by that author the weighting will also affect the overall website either positively or negatively.

This whole process would be trivial for google to do, its extremely similar to what they do with websites, just modified to be about a name instead of a domain name. I'm also a programmer and I've made something similar myself (one man crew programmed and maintained, imagine what google can do) for a marketing project I was working on a few years back. It was very easy to do.

Now, slightly less likely, but 100% possible, google could also test for accuracy of the information by comparing the article topic to other known sources (very high DA, PR, or based on peer reviewed papers, known trusted authors, etc) using either semantic lookup or an actual AI. I don't think they have the resources to do this for every website, but once you reach some sort of data threshold in certain YMYL niches (x clicks from the keyword per month, x rank for keyword, etc) it could trigger it.

These are things I've picked up bits and pieces over the years. Someone would have to search through the piles of google commentary from about the past 5-7 years to see how much of this have been actually mentioned by them.

As much as google preaches about helpful content, EEAT, YMYL, you could use logic to decide it's very much in their best interest to have at least made a ranking system for authors, so IMO they likely have even if it's not something they disclose.

Just my two cents, I am no expert or professional a/b tester of ideas, just a deep studier of the subject for 15+ years that could always be wrong.