On the web site, you select coordinate format and datum as a unit. There are limited choices. In particular, the only datum available for UTM is WGS84. NAD83 (which isn't all that much different for North America) is available for USNG. Account tab, Units sub-tab.
The Explore app allows independent selection of coordinate format and datum. It has a much, much larger selection of datums. Including, for example, NAD27 CONUS, which is required for UTM coordinates in conjunction with old-school paper USGS topos.
OK, I never would have found the units change without advice to look in Account, thanks.
You are correct the choices are very limited and are a problem, we are using GDA94 here in Australia and it is more than 100M different to WSG84 which is the nearest choice. Which is as I suspected because the locations correctly located in Explore App iOS are all out of position by the same amount when looked at in Explore Web.
That is all a bit poor, Grid selection is pretty fundamental to gps / mapping / location.
