If you are getting "xdrx.XX.XX.arx cannot find a dll or other file that it needs." you'll need do one of these steps:

A) copy XDRx.x64.dll for 64bit or XDRx.x86.dll for 32bit system into the same folder where acad.exe is
B) add full path to XDrx-API/SYS folder in AutoCAD Options -> Files -> Support File Search Path

P.S.
Can we please standardize the naming of this API? The forum's section is XDRX, topics are XDrX, the git repository is XDrx, the api files are XDRx

And just while we are on the subject. If this modifies any system registry, environmental PATH, or copies any files beyond its folder, please provide detailed uninstall instructions.

The special situation you encountered,
Under normal circumstances, there is no need to copy any files to other places.

Only path and trustedpath paths are set

It seems that AutoCAD is buggy with this code... In BricsCAD it alignes UCS to region and in AutoCAD it reports that this object don't define coordinate system... And to be even more buggy it doesn't remove created regions after solidedit - copy - face options...

Thank you, I learned a lot of information from you.

And just while we are on the subject. If this modifies any system registry, environmental PATH, or copies any files beyond its folder, please provide detailed uninstall instructions.

FYI, my recommendation “path to %PATH% with SetEnvironmentVariable” doesn’t stick
It’s just for the process

https://learn.microsoft.com/en-us/windows/win32/api/winbase/nf-winbase-setenvironmentvariable

SetEnvironmentVariable
Sets the contents of the specified environment variable for the current process
This function has no effect on the system environment variables or the environment variables of other processes.

Look, I am using BricsCAD and whereever I pick point on 3dsolid - BOX my both X and Y axises are aligned to edges of BOX... Haven't tested it with AutoCAD - give me a minute to try...