I received this response from Salesforce support:
On my research, Salesforce Template-Driven Communities have been build
with a responsive design. So, depending on the size of the browser
window, the layout will be adjusted to fit best on the screen.
• The navigation menu, will show a "More" button on the right, if not
all menu items will fit within the width of the screen. • Or when you
resize your screen to the size of a mobile, the navigation menu will
collapse into a Hamburger icon on the left hand side of the screen. •
When you click on the icon, an overlay will slide in with a vertical
menu instead of the horizontal menu.
• Depending on the size of the screen, a Dashboard will show multiple
columns and rows. • Dashboards on a Mobile will present all components
into one column. • As part of the responsive design, some components
and buttons might be hidden on smaller screens. • Chatter posts
created in Desktop mode can't be edited when in Browser mode.
With the information in mind, there is no specific device
specification, it may be different on each type of mobile device and
size of the device. For reference you may also check our article about
"Community does not show all features in mobile browsers".