From Olga via Carol:

Generally, our preference is for tablets to get the mobile experience as we assign based on operating system. The only exception to this is for the new iPadOS, which redirects all sites to the desktop experience. All previous iPads are still on iOS and receive the mobile site

Current definition

We use the User Agent to define whether we assign mobile or desktop. The UAs we consider to be mobile/tablet are captured here: https://github.com/wikimedia/puppet/blob/cdc9e51e7a3ed2e4f122aba692f1eaad52b57fa4/modules/varnish/templates/text-frontend.inc.vcl.erb#L103-L104

This information should be added to https://www.mediawiki.org/wiki/Wikimedia_Product/Data_dictionary#Readers.