Does the already installed antivirus protection protect the Mac from ransomware?
I have been running antivirus software and thinking of stopping because of the built in protection. But haven’t seen anybody anywhere say that the built in protection will protect from ransomware.
Ransomware is just another form of malware. So yes, the macOS built-in protection (XProtect) would be a good level of protection against that or any malware.
Besides that, any malware needs a way to get on your Mac. The primary way it happens, almost all of the time, is someone gives the "ok" and installs something that is malware or has malware as part of it. This happens when you download software from sites you shouldn't trust.
Notice I say "sites" not "software." It is the site that is the most important. You can trust an app, say Photoshop, but if you download Photoshop from a site that isn't the official one, maybe one promising a free version, then that's how you get malware.
So always trust the site you are downloading from. If you don't, or aren't sure, don't do it. Also, limit your exposure by downloading only software you really need. Don't just download things here and there because they look interesting or fun. That's how a lot of people get into trouble.
And, of course, back up your data. That's another layer of security.
As for third-party anti-malware apps, I'm not a fan. They often lag behind Apple in adding protection against new threats. They boost their stats by claiming to protect your Mac against things that are only dangerous to Windows machines. And often they put up alarming "maybe" results or privacy warnings that make you think they are doing something so you keep paying. And they break apps and web sites by overprotecting you and disabling functionality. Not to mention that can slow down your Mac too.