A lot of mysterious executable files inside /Users/Shared

Note: this doesn't answer your question directly but is advice and some further steps to troubleshoot.

If your computer has been reported as being affected my malware, you are likely better off long-term to back up your personal data (i.e user directory etc), note the apps you have installed, and start over with a nuke & repave. A few hours spent doing this is better than rescuing a system with unquantified levels of malware -- it could be just 'SystemExtr' or it could be a whole heap of other things -- and the nature of some malware means it can't be 100% removed.

In the meantime:

• use a reputable macOS malware scanner like Malwarebytes or Sophos to see if they pick anything up;
• upload one of the executables to VirusTotal to see if it find anything;

And for ongoing homework: learn a bit about hard drive space management and reduce reliance on the whizzy apps (especially the free ones) that do all this for you. Some are reputable, some are junk. Using a disk viewer like Grand Perspective will give you better visibility into what's going on inside your hard drive, and give you more control overall.