NEW INFORMATION: I still stand by my opinion (now in the section prefaced with "MY ORIGINAL ANSWER"ANSWER (trimmed down)", below) that Ubuntu should offer both snap and non-snap versions of its packages instead of forcing us all to use the snap versions. However, I also discovered how to bypass the containment restrictions of the existing snap-based package:
If the standard Ubuntu snap-based chromium package is installed, we can run the program like this:
/snap/chromium/current/usr/lib/chromium-browser/chrome [ ... args ... ]
This bypasses the containment restrictions.
MY ORIGINAL ANSWER (trimmed down):
I understand that Ubuntu has given us the new-and-improved feature of forcing us to only be able to download a snap-based version of chromium from the standard repos. And as an extra, added bonus, I now understand that we cannot access data from separate, directly referenced filesystems from this and most other snap-based software (yes, I am being facetious in giving this "praise" to Ubuntu).
Because of this,(Editorializing deleted) I have followed a suggestion in the following discussion which explains how to disable the crippled, snap-based chromium and to install a version which offers full capabilities, including the ability to access the filesystems which I want to access:
How to remove snap completely without losing the Chromium browser?
I didn't remove snap completely ... all I did was perform the part of this that was suggested by the user called "eitch" (around 75% down towards the bottom of that discussion). Using this non-snap version of chromium solves my problem.
Another solution to this problem might be to abandon Ubuntu altogether, and to switch to Linux Mint, given their decision to not offer snap software in their repos and making it so users specifically need opt in(Editorializing deleted) to installing snapd and snap-based packages. Because of this snap issue, I'm now strongly considering giving up on Ubuntu and moving to Linux Mint or some other distro. I'll decide soon.
UPDATE: Yet another solution would be for Ubuntu to offer both snap-based and non-snap-based packages for software such as chromium, so that each of us users could individually decide whether we want the full, traditional functionality of the given packages, or whether we want to install snap-based versions of the packages which make use of containment and might therefore have lesser functionality than the older versions. Of course, this solution is not available to us unless Ubuntu decides to change its policies concerning snap. Don't hold your breath waiting for that to happen.(Editorializing deleted)
ANOTHER UPDATE: I have now investigated this behavior under flatpak and appimage, and programs that are run via these services seem to have no trouble accessing all of the partitions and files on my system that are accessible via the command line. This containment restriction which cripples chromium apparently only applies only to snap.