The volunteers said the names of the veterans aloud as they placed each flag at the individual graves, a gesture of respect for their service.
Among those who participated in the tribute were two young siblings whose mother is a veteran herself.
"It's just a big deal for me and my family that we pay respects to people who have served, to save your lives, for this country and for others as well," 10-year-old Brooklyn told ABC7.
Her brother, 11-year-old Benny, said: "They've died for us and they served the country, protecting our lives when they're fighting and we're having fun here. So we want to put each flag on each veteran."
Watch ABC7 Salutes Memorial Day 2021
The flag placements were organized by organizers with Honoring Our Fallen, an organization that helps the families of military personnel who died in combat.
"We came out in 2012 and we place 26,000 flags with about 300 people that first year," said Brennan Leininger, a spokesman for the group. "Over the years, just slowly we've been getting more and more community members involved. And now we have several thousand people that come out and we put over 225,000 flags out."
Last year's event was canceled due to the coronavirus pandemic. This year's event was modified, without the usual ceremony and flyover tribute.
Nevertheless, individuals and groups spent Saturday morning covering the nearly 13,000-acre cemetery in a sea of miniature American flags.
"I think it's really actually very special that we get to do this," said Gabby Sarkis, a Girl Scout who lives in Corona. "It helps us remember everyone in the world, the people who served and did all of this stuff for us."