It’s a group of idiosyncratic people so there are not hard guidelines but there’s definitely a hierarchy of who they are more likely to describe as advance civilizations and who they question were capable of major engineering projects. This can bring out really weird stereotypes: I’ve heard people make arguments where they positively describe American Indians as living in harmony with nature, etc. but question the estimated populations, trade networks, etc. for e.g. the Mississippi tribes from what appears to be a mix of “noble savage” mythology and sort of mentally having slotted them into a larger model which made it easier to doubt one detail than reconsider their larger intellectual framework. The “aliens did it” people are far out on that spectrum but less extreme versions aren’t uncommon.