What you are looking for is not desktop mode but kiosk mode. I only know how to do do that in Chromium based browsers (like Chrome and Edge). It won't block other programs, but prevents them from showing when the kiosk has focus.
I have two simple examples:
1. using TProcess (not written by me):
Your code is Good solution for C:Q2 and C:Q3 , great code,(enough for Me).

Thank you.
I solved both C:Q2 and C:Q3. bellow, thanks Thaddy , (I dublicate here..)

optionally=
C:Q2=How to detect the my win10desktop is "BrowserMode"?
Here I can calculate the sum-opera.exe activity.
C:Q3=Can you give me, other/s a list of win10dektop-Mode?
Perhaps you need some examples from me, examples of win10kiosk-Mode.
200=IF I start SolidWork.exe then My entire pc+desktop is win10kios-Mode=Tech Mech Top-power of -Mesh.
201=If I run RedAlert3.exe and games with very-power-of-opengl, then
win10kiosk-Mode=Opengl-Games3Dmode.
202=If I wrote one DVD-R with 4.7Gb, then win10desktop-Mode=DvdWrote, and
all others/ application must "blocked" by the user.