This all depends on what dimensions / definitions you are using....
From the standard "Device" and "Device Type" perspective, if the user is on a mobile device it will record as that... and if you definition uses those dimensions, then it will also be treated like a mobile visit.
In my own implementation, I use the standard device tracking as above, but I also have a custom eVar tracking the "responsive breakpoint" of the site. I know that the site has set display breakpoints at specific screen widths... so I have a simple script:
var breakpoint;
if (window.innerWidth <= X) {
breakpoint = "mobile";
}
else if (window.innerWidth >= Y) {
breakpoint = "desktop";
}
else {
breakpoint = "tablet";
}
return breakpoint;
Replace X with the maxium pixel width that designate the mobile experience (i.e. let's say 750 for example), set Y with the minimum pixel width for the desktop experience (let's say 1000 for example), then anything that is between the two will be caught by the else for tablet experience....
If you have multiple sizes such as portrait mobile, landscape mobile, portrait tablet, landscape table, small desktop, large desktop, etc... you can add more clauses to check "else if (window.innerWidth >= W && window.innerWidth <= Z)" to check for additional widths....
Then you can create reports by device or reports by the "breakpoint experience".