Defender is written and constantly updated by Microsoft themselves.
They know their own Windows source code intimately and it is in their interests to keep their operating systems viable and virus free - to maintain their market share, so they keep a close eye on malware and nip new viruses etc in the bud
I was told that owing to competition rules, Microsoft are not allowed to advertise Defender. Third parties will try to demean it because they want to get money out of you, but Defender is free and perfectly integrated with the Windows operating system. It also automatically updates your machine as new threats come along, and does not chew up memory or CPU time or cost any money, so is a good thing to have.