Let me start by saying that I tried to find credible research, any proper research that shows that beginners indeed tend to choose "over" rather than "under". Unfortunately, I've only found anecdotal evidence (not to offend anyone, it's just a term for evidence based on one's own observations and experience).
But others have explained why they or someone else can choose "over" more often than under, and the arguments make sense.
I also didn't find any research on people who bet on the under and over goals market, but I found this very interesting article:
https://tomcampbellwatson.medium.com/no-one-wants-to-bet-the-under-2c5ec4d14c75People tend to bet on the over and not the under, as described in this article, and I have seen many prediction sites that, when analyzing games and are unsure which team will win the game, they choose to say that the prediction will be over 2.5 goals or over 3.5 goals. Now, if they are beginners in sports betting, I highly doubt that they will bet on the over and under market.
I say this because when I started with sports betting, I didn't know anything about over and under or handicap, I just bet on 1x2 and I believe that this happens to more than 90% of people when they are starting out in sports betting.