Racism is not an isolated incident, but rather, an interwoven part of everyday American life.
This is rooted in the reality that from the founding, and through the formative years of American history, being white was a requirement for American citizenship and all of its associated rights or privileges. That was racist. The consequence of this, is that society developed in a way that naturally operates to the advantage of White people, independent of anyone’s intentions.
For example, you can pass a law that makes it illegal to segregate schools by race, but that doesn’t really matter if all the White people choose to move away from the schools with Black students. It doesn’t really matter why those people are moving away, the effect remains the same.
This is what it means when CRT theorists say “racism is endemic”