It seems like people in the low-risk tier might be more likely to get J&J. Firstly, it's just the one shot and secondly, people who are worried about COVID are vaccine shopping because they want the perceived "higher efficacy" of the mRNA vaccines vs J&J (whereas lower-risk people might be more interested in vaccine passports than preventing symptoms).

Maybe. In a lot of places you don't really have a choice and, in the US, AFAIK the mRNA vaccines are more common. Also, while "vaccine passports" have started being discussed, they're not really a factor yet--given how many people still need to vaccinated--and may never be outside of scenarios like schools. (That said, I have heard people who see getting a vaccine as more pro forma saying they prefer J&J because it's just a single shot.)

It's pretty easy to "vaccine shop" (if you care about it) at least in the US with all of the scheduling being done online. Sure, if you go to one of the big vaccination sites, you may not get a choice, but it seems that sites offering J&J have been advertising that and ones that don't indicate seem to be Pfizer & Moderna (or if J&J, give you a choice, at least around Massachusetts).

J&J is also widely used in the US for people who might have issues with scheduling a second shot, for example people who are homeless, or are home-bound.

Fair enough. It was still quite hard to get one when I scheduled and I was going to take whatever I could get even if I favored the mRNA ones. That said, if I didn't really care about getting a vaccine but was going to get one anyway, I'd probably just choose whatever I could get most easily.