how to reinstall MacOS after malware infection

Here’s a guide which will reinstall the latest version of macOS that is currently installed on your system: How to reinstall macOS - Apple Support

Video guide from Apple Support: https://youtu.be/HCrl_4k0aqo

EDIT: If this doesn’t get rid of malware, you’ll need to factory reset which wipes everything on your Mac by following Erase all content and settings on Mac - Apple Support

There is Adware & Malware which will affect the computer and is generally downloaded from “ Shady “ Websites or Developers who Inject this software into the download.

Should “ Certain & Specific Software “ referenced from above be installed - it will negatively impact macOS. It is suggested, to download directly from the Developer, the application Malwarebytes for Mac. 

It is free or paid for added features. 

Run the Application and it should remove the malware / adware. Once done, restart computer and test.

What malware did you install?

How do you know you installed malware?

My questions are because there is no reason to reinstall as nothing can alter the System volume. There is likely a very simple solution.

You don't wipe a drive and reinstall the OS just to get rid of malware. You run an Anti malware app like MalwareBytes or DetectX Swift.