Participation is required in recitations and workshops which are both online and synchronous, so take 6.UAT only if you will be able to participate in them. As for lectures: L1 is online and synchronous and recorded for later viewing; L2-L6 are prerecorded asynchronous. On-campus students may have the option to participate in-person office hours and a few in-person workshops. Check uat.mit.edu (after F20 ends) for any additional information - e.g. this link was specific to F20, there will likely be something similar for S21.