LOS ANGELES — Jurors in a landmark social media case that seeks to hold tech companies responsible for harms to children got their first glimpse into what will be a lengthy trial characterized by dueling narratives from the plaintiffs and the two remaining defendants, Meta and YouTube.

At the core of the Los Angeles case is a 20-year-old identified only by the initials “KGM,” whose case could determine how thousands of similar lawsuits will play out. KGM and the cases of two other plaintiffs have been selected to be bellwether trials — essentially test cases for both sides to see how their arguments play out before a jury.

Comparing social media platforms to casinos and addictive drugs, lawyer Mark Lanier delivered opening statements Monday in the Los Angeles Superior Court trial that seeks to hold Instagram owner Meta and Google’s YouTube responsible for addictive features and harms to children who use their products. Two other defendants, TikTok and Snap, have settled the case.

Meta lawyer Paul Schmidt spoke of the disagreement within the scientific community over social media addiction, with some researchers believing it doesn’t exist, or that addiction is not the most appropriate way to describe heavy social media use.

Luis Li, the attorney representing YouTube and Google, delivered an opening statement on Tuesday focused on KGM’s user data, saying the five-year average of her watch time is 29 minutes per day. He said KGM’s average daily time spent on YouTube Shorts, watching vertical short form videos with the “infinite scroll” feature Lanier called into question Monday, was just 1 minute and 14 seconds.