I don't know about flat-earth specifically, but lots of people only believe a few conspiracy theories. There are lot's of "spherical earthers" who believe the moon landing was faked for example. A very large fraction of the US believes in at least one of the major conspiracy theories associated with QAnon (several of which predate QAnon), but a much smaller fraction is actually involved with QAnon.
Sure, there’s no critical thinking for a lot of it, in the same way that I don’t think critically about all the physics and engineering before I step on an airplane. The entire point of living in a society is that you don’t have to do everything yourself — you can outsource a lot of that critical thinking to other people who (are paid to) care more than you. You can just go with the flow and actually have a pretty good track record for believing the right things.