How to reset Mac OS Base System?

I believe I am a victim of a spyware both on my Mac and now on my phone through MDM. I have factory reset my MacBook Air multiple times but I believe the root of the problem is in the base system of my computer. After a fresh reset of the hard drive and turning on Firewall, the programs are already compromised, like Safari where it allowed the download of a modified Zoom software; or the AppStore where I got the Clean My Mac which was bugged as well. This I know because the apps have a series of letters and numbers in front of it. Looking through the Library, I soon found programs about MDM, “Supervisor,” among others when my Mac is a personal computer of 7 years. I would have taken screenshots of the files but I’m just exasperated. I know it’s old but I never had a problem with it until recently.

Anyone has the answer?

Thanks!