SAN FRANCISCO — Jury selection is expected to begin on Wednesday in Los Angeles for another landmark trial about the alleged harms of social media on young users, with Meta and, for the first time, Snap as defendants.

Thousands of lawsuits have been filed in recent years in local, state and federal courtrooms across the country with various claims about alleged harms to children and teenagers caused by social media companies.

The cases — the first of which went to trial earlier this year in Los Angeles — have taken twists and turns.

Meta and YouTube lost the first so-called bellwether case earlier this year, when a jury found them collectively liable for harming a young user known by the initials KGM, and ordered the tech companies to pay a combined $6 million penalty.

Meta said at the time that they “respectfully disagree” with the verdict.

Snap and TikTok settled with KGM ahead of that trial.

Months later, Snap, TikTok and YouTube settled another case that was set to go to trial in August; that plaintiff, known as RKC, then withdrew their remaining claims against Meta, allowing the Instagram and Facebook maker to avoid trial without a settlement.

TikTok and YouTube were previously named as defendants in the new case but both companies reached settlement agreements with the plaintiffs, known by their initials SJ and KDB, before the start of the trial, their lawyers said.