As vikram360 pointed out, they're on the schedule page (https://pdos.csail.mit.edu/6.824/schedule.html). Since this is the class going on right now (Spring 2020), only a couple that have already happened have been posted so far, but I'd bet they're uploaded pretty soon after their date.
You can also find the Youtube playlist here: https://www.youtube.com/channel/UC_7WrbZTCODu1o_kfUMq88g