I am a beginner in Linux, so some things are strange and new. For example, the behavior of the cursor. When I move the cursor over, say, the browser, it appears correctly, but when I move it over the top bar or the dash bar, it changes to the default cursor. Why is this happening?
The cursor is the Oreo custom theme and it is located in home/.local./share/icons.
Z18C stock
Are you on Zorin 18? If so, based on my own experience with customizing the mouse cursor and this post that was recently made in the forums, it seems like it may be an Zorin 18/LibAdwaita issue that is not easily resolved. I'm keeping an eye on the forums just in case there is a resolution.
There is no easy resolution.
LibAdwaita is locked down, tight. And - Gnome set it as a dependency for many applications and certainly for Gnome Desktop and Gnome Shell.
LibAdwaita is not a library that runs applications.
Rather, the only function it serves is as a lock. Portions of GTK that control widgets were removed from GTK and placed into LibAdwaita.
LinAdwaita hides the .css and other assets that control their appearance, look and accessibility options, overriding and injecting Gnomes preferences over the users.
There are some problematic workarounds, like LibAdapta (Mint) or LibAdwaita-without-LibAdwaita (Arch), but these are incomplete and prone to breakage... partly because Gnome strikes back with updates.
We left Microsoft... only to end up being controlled by Gnome, just as vigorously.
Thank you for the information. Based on Google searches ("libadwaita cursor issue"), it seems to be an issue that’s several years old. Maybe more of a feature. As a beginner, it’s better that I don’t try to fix it at this stage. I’d just end up messing something up.
"We left Microsoft... only to end up being controlled by Gnome, just as vigorously." Well said 
Interesting, that would explain why after installing Plasma, my yellow cursor changes to a black one when moving from Desktop area to selected icon.
This could be because You have put the Cursor Theme in the home Path. Try the following:
Switch Your Cursor Theme to the Default. Then move Your Custom Cursor Theme from the home Path to /usr/share/icons/ and then reboot. After reboot, choose Your Custom Cursor Theme again.
This would be a separate issue, actually and one I believe you can control through settings; but I am not familiar enough with Plasma to provide details on which settings and where to find them...
That did the job. Thanks! 
Can we therefore mark Ponce-De-Leon's post #6 as 
Solution ?
I've marked it as Solution.
Glad to see that this was the issue on this. 