"Things reached a tipping point, we had to come out," Yang, 32, who declined to be identified by her full name given fear of reprisals, told Reuters.
Authorities have denied the deaths in the fire were linked to lockdown measures that blocked the victims' escape.
"I'm very proud that I can stand up with the best young people in China and speak out for everyone," said Cheng.
She and other young protesters are tech savvy, with many communicating over Telegram in amorphous, anonymous and decentralised acts of defiance, with echoes of Hong Kong's leaderless pro-democracy protests in 2019.
But it's better than facing the reality day by day and then not being able to do anything, and then you feel sorry for yourself."