New Mac mini (2018) and multi-monitor desktop

With my previous Mac (iMac 2012 running Mojave) and two external monitors attached, 1) the desktop was really one wide desktop (open windows (browser or spreadsheet or...) would span the monitors, part of the window showing on adjacent monitors, and 2) when I used the four-finger swipe on the trackpad to change to a different desktop, all three monitors changed desktops. Now with the Mac mini (also on Mojave) windows are only shown on one monitor at a time (I can move them from window to window but they will not span across two monitors). And, the four-finger swipe only changes the desktop on the monitor that contains the cursor.

Apple Care basically said "oh well". So I'm just checking to see if others have been able to get spanned windows and all screens changed on the swipe. Thanks.