I hope to be near a dev computer in a couple of days and then I can help. 
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It is not true for macOS prior Catalina. It is not true for Windows 10 <= 1904 (Pro Edition).
Now, the last Windows 10 releases may exclude 32 bits applications runā¦
It may not be supported
what does
dpkg --print-foreign-architectures
list ? it should be a list of all the architectures dpkg is configured to install packages for
pi@raspberrypi:~/Documents/RP-Tunnel (Andy) $ dpkg --print-foreign-architectures
armhf
i386
OK so you can install packages for armhf which is ARM 32 bit
i386 makes no sense for a Pi
that command doesnāt add any packages
it just told dpkg to also permit armhf (32 bit arm) packages to be installed
youāll still have to grab those using apt-get
Copy. Iāve tried a few things, like:
pi@raspberrypi:~/Documents/RP-Tunnel (Andy) $ sudo apt-get install libgtk-3-dev:armhf
Reading package lists... Done
Building dependency tree
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:
The following packages have unmet dependencies:
libgtk-3-dev:armhf : Depends: gir1.2-gtk-3.0:armhf (= 3.24.5-1+rpt2) but it is not going to be installed
Depends: libgtk-3-0:armhf (= 3.24.5-1+rpt2) but it is not going to be installed
E: Unable to correct problems, you have held broken packages.
pi@raspberrypi:~/Documents/RP-Tunnel (Andy) $ sudo apt-get install libgtk-3-0:armhf
Reading package lists... Done
Building dependency tree
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:
The following packages have unmet dependencies:
libgtk-3-0:armhf : Depends: libcups2:armhf (>= 1.6.0) but it is not going to be installed
Depends: librest-0.7-0:armhf (>= 0.7) but it is not going to be installed
Depends: libsoup2.4-1:armhf (>= 2.4.0) but it is not going to be installed
E: Unable to correct problems, you have held broken packages.
etc. etc. etc.
Each package depends on other packages. Is there no command to say āinstall this and all itās dependencies!ā
welcome to linux 
sometimes there is a UI for the package manager that handles all of those dependencies that have dependencies for you
As far as I know doing it manually on the cmd line means you get all that joy of sorting out all the dependencies as well
I could well be wrong and there is some other incantation to issue that will recursively install all the dependencies
am using the Pi 4 with 32 Bit Raspbian because there are issues with 64 Bit raspbian. Missed 32 bit libraries for example. The is only helping: WAIT and have PASSION. It is still not complete for 64 Bit.
Sorry for the delay in responding, and the short answer is that Raspberry Pi ARM 32-bit apps should fail on 64-bit versions. The reason is that Linux has either a 32-bit OS or a 64-bit OS. There are programs that attempt to combine the two operating systems together, which seem to have mediocre results. I have not tried them myself.
For now, the only option with Xojo and Raspberry Pi seems-to-be running a 32-bit Raspberry Pi OS for 32-bit Xojo apps.
The reason 32-bit apps run on a Windows 64-bit OS is because Windows 10 has both 32-bit and 64-bit operating systems installed at the same time. 32-bit Window apps run with libraries from C:\SysWOW64, and 64-bit apps run with libraries from C:\System32 directory.
I hope this helps 
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On Linux simultaneous support for 32-bit and 64-bit apps seems to be named multiarch (just found it online, my experience with Linux is very limited and I have almost no experience with the RPi).
Here is some information on multiarch for Debian (Multiarch/HOWTO - Debian Wiki) which is not specific to the RPi, so it may not apply here (you could try the commands listed there though, and see what happens).
And here is another link with information for multiarch on the RPi for Ubuntu: https://wiki.ubuntu.com/ARM/RaspberryPi#Multiarch
HTH,
Julen
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@npalardy was trying to lead us in the direction of multiarch, but so far we havenāt found anything that successfully works.
Probably just going to continue on 32-bit for now, and see if thereās a way to recompile other 64-bit app into 32-bit. Donāt really have the time that itās going to take to get Xojo working on RPI 64-bit.