As you already have been told the antivirus protection that comes with Windows 10 is Windows Defender and it is active by default
As you also already know, you should never run two antivirus programs on the same machine because each generally thinks that the other is a threat based on the methods they use to do their jobs. Essentially an antivirus program often looks like a virus to a second running antivirus program. They then attack each other and engage in a fight to the death, so to speak, usually hobbling your computer.
It is up to you what antivirus program you wish to use. If you install a third party antivirus like Avast I do not know of any of them that do not disable Windows Defender as a part of their own installation process. Most of the better ones will actually re-enable Windows Defender as part of their uninstall should you decide to uninstall the third-party antivirus.
I've used a lot of antivirus programs over the years, and can count on less than one hand the number of infections any one of them has caught. That's mostly because of my web browsing hygiene, as I call it. If you have looked at the virus quarantine locations in whatever antivirus programs you might have been using over the years, and don't generally find anything there, then any decent antivirus program will be more than sufficient. If you find that the virus quarantine locations have been well-populated, then you need to be particularly focused on getting an antivirus program with a very high detection rate that doesn't go over the edge into detecting things that aren't viruses as if they were (and that has happened with some products). From my perspective, if you have a lot of quarantined material, you'd be better off doing a thorough examination of both your browsing habits and what browser-based extensions might help you to avoid infections in the first place.
I worry a lot. . . I worry that no matter how cynical you become it's never enough to keep up.
~ Trudy, in Jane Wagner's "Search for Signs of Intelligent Life in the Universe"