The build in EPP or (Endpoint Protection) product in Windows is called Windows Defender. Are other programs necessary I would say that depends. Breaking a security product down into three categories on whether its good or not you have Performance, Real life protection and malware protection.
Performance is just how much does the product impact a system while it's protecting the system and user.
Windows Defender ranks pretty low in this area so expect to have performance impacts (mainly due to scanning)
Real life protection - This part has to do with protecting the user from things like malicious websites, phishing attacks. Think attacks against humans rather than a computer system. Windows Defender is middle of the road in this category.
Malware Protection - How well does it stop or prevent attacks. It does some in-memory protection which is good and it does a good job of stopping threats before and during execution.
One caveat to mention is that Windows Defender is one of the only EPPs that can be disabled via GPO even with tamper protection enabled. This is bad and threat actors know about this and do leverage it to disable protections.
For an average user yeah Windows Defender may be adequate but be mindful and possibly do some free security awareness training online to help protect yourself against social engineering attacks.
As a security professional I don't rely on Windows Defender and typically run security suite products as they tend to offer a lot of additional features particularly in the real life protection area.
One mention I will give is Bit Defender is consistently considered a well rounded effective EPP. It's not the only good one.
This is a great resource of independent EPP testing with detailed reports for different products.
Not sure which antivirus software works best for you? Following our in-depth comparison of antivirus and antimalware tools for Windows, Android, and Mac.