Is there a way to update virus definitions, or is it done automatically with updates?

If you are referring to the anti-malware software embedded in macOS, then it is updated automatically by Apple as changes occur and, also, during updates or upgrades, if needed.

If you are referring to third-party malware protection, then you need to check their documentation or websites for that information.

FYI: These are user-only forums. This is not Apple Support. if you need to contact Apple Support, then contact product and tech support:  Contacting Apple for support and service including international calling numbers.

These forums only provide help with Apple products. Thank you for your understanding and support.

You mean the OS itself? That is done automatically anytime you're online. Gatekeeper keeps itself updated.

That said, Gatekeeper watches for known Mac malware and attempts to stop you from installing or launching it. Not that you still can't do it if you choose to ignore two or three warnings. I don't believe it does anything to watch for Windows malware since none of it can do anything to a Mac.

Not infecting Windows users from your Mac is very simple, and is done by following two simple rules.

1. Never forward email attachments to your Windows using colleagues. Especially when you don't know why you got it, or don't recognize the name of the sender. The latter can be very easily spoofed, so recognizing the name doesn't mean it's legitimate.
2. Never run a Mac app sent as an email attachment you weren't expecting. There is some (very little) Mac malware distributed this way.

If this is the company's Mac, you'll have to comply with their rules. They want to, not surprisingly, protect their systems from being brought down by some of the very invasive malware out there. If it's your Mac, make them happy by installing non-invasive AV software, such as ClamX AV. By that, I mean it can be run on demand and not taking up system resources the entire time your Mac is on. It used to be free, but is now $30 per year for home use.

Be aware that at this time, and still, AV software is to help you protect Windows users from getting malware from you. For yourself, it's entirely worthless.

If your company does not mandate that you must use a Mac anti-virus application, then there are two things to know: 1) Viruses have and continue to infect MS Windows, and 2) there are no viruses on Macs unless one hitch-hikes in from a shared Windows originated Microsoft document. The latter still cannot infect the Mac, but the virus can be passed along to the next Windows user of that document.

If your company does mandate the use of a Mac anti-virus application (as some do) then they should be paying the subscription for it, and you should routinely check for updates from the application. We do not however, recommend using any anti-virus product on the Mac as they impact performance, browsing, produce false positives, and interfere with the built-in operating system security.

Show these articles to your management team:

Security, Built right in.

Apple Platform Security.

System Security Overview

Apple issues periodic anti-malware updates (usually monthly) for Macs running a supported version of macOS in its sliding, three-year support window that presently supports Catalina, Mojave, and High Sierra. These are automatically applied if your System Preferences : Software Update : Advanced… panel [√] Install system data files and security updates is selected.

Both Windows and Macs are susceptible to unsecure downloads from non-vendor sites of adware and other forms of malware, and for that we recommend the free Malwarebytes for Mac. It is not an anti-virus tool, but is purposedly designed to detect and remove ad/malware.