lencw on "[Plugin: Lazyest Stylesheet] What triggers the mobile stylesheet"

I have been using this excellent plugin for well over a year now, it saves lots of time messing with theme files, many thanks.
What is not clear is how the mobile stylesheet gets invoked. For a Samsung Galaxy phone, it is clearly using the mobile css file, but using texting sites such as MobileTest.me and checking the iframe there is no reference to the mobile file in the page header for the website.
I have used @media
only screen and (max-width: 760px),
(min-device-width: 768px) and (max-device-width: 1024px)
in the standard stylesheet, and this appears to work on the mobiletest.me but I am not sure what happens on the actual device such as an ipad.
I need to know which file gets invoked on which devices, can you please post this to the plugin page it would be a great help.
Thanks

https://wordpress.org/plugins/lazyest-stylesheet/