That would depend on the app. Malwarebytes, for example, will install the full version, which does automatic scanning, as part of the free package. However, the automatic scanning is turned off after a time (30 days? 60 days? 90 days? I don't remember. I do remember that you get a warning that if you don't move to the pay version, auto scanning will be turned off.) During the period that autoscanning is on, users get automatic updates of the malware detection library, so the threat environment status is updated. If the pay version is not activated, the auto updates stop. After the initial 30-60-90 days, the free version no longer automatically updates its antimalware library and no longer automatically scans. Users who want those features are expected to pay. It will scan and update the library on demand; if a user launces the app, it will ask if the user wants to go to the pay version, then the user can scan with the existing antimalware library, can update the library and then scan, can just update the library, or can just quit the app.Ok tell me this why companies has a paid version and a free version??
Other antimalware apps may have other business models. Contact the vendors, they will be happy to explain in detail why you should buy their product. As the current threat environment on Macs is such that antimalware is not necessary, my credit card stays in my pocket. YMMV.