Which OS is this?

The first rules out Linux (unless this is a SELinux thing?).

The second rules out Windows/Mac OS/Linux with X11.

The upstream KeePass application does not support mobile operating systems. It barely supports non-Windows, which is why when most people say they use KeePass they mean KeePassXC

In Linux you can't attach to arbitrary processes depending on configuration. You can start a program with gdb, but not attach to a running one by default. It was a security upgrade a few years ago https://www.kernel.org/doc/Documentation/security/Yama.txt

Maybe NixOS? I've barely played with it, but this seems like the level of isolation/paranoia that NixOS seems to offer (as far as a layman like myself understands - could be completely wrong on all counts, as usual).

I think you might be misunderstanding nixos, the tree separation will not improve security for this kind of scenario, the only guarantee that nixos offer is that the package is exactly as it’s described on nixpkgs if the store is untainted.

Wouldn't be shocked at all, I mostly think of it as "Linux but with a bunch of inherent security separations introduced", which is why I thought it might be what the...grandparent at this point? is using.

gotta love how every discussion pertaining to program isolation in the last 20 years is just "well all these years i thought this thing isolates processes like it ought to do", followed by a quick, "nope, it actually doesn't". i blame UN*X

Eh, it's a field/domain I am definitely ignorant of - at least I'm not out there confident in my wrong beliefs X)