Do they use 'off-the-shelf' passenger jets?

I had to Google this one, but yes, it looks like an A340 has been used in Antarctica recently [1].

[1] https://www.independent.co.uk/travel/news-and-advice/antarct...

I'm impressed. They got four APUs to take off from Antarctica? ;)

I suspect the biggest concern operating off of an earthen runway is ingestion of FOD. For unimproved runways the 737 Classic could be fitted with so-called gravel kits to reduce the chances of ingesting crap from the runway on takeoff. That's why you still see 737-200s in service in the wintery parts of Canada – the fans on the NG and MAX (and A320) are too big to make gravel kits practical.

However, where the weight itself is an issue, Indian Airlines (now Air India) ordered some A320s with beefier landing gear precisely to operate something that heavy off of less than great paved runways.

Ural's not going to have much in the way of manufacturer support though so their options are a bit limited.

The Kiwis sometimes fly a 757 to McMurdo (see e.g. https://www.stuff.co.nz/national/3173536/Getting-there-in-ha... )

And "normal" C-130s and C17's can land too (I've been on both to McMurdo...)

But this is a packed snow runway. Williams Field is for ski-equipped aircraft only, I believe.