When does Tomcat add and remove threads?

[John.E.Gregg]

I am using Tomcat 9.x.

When does Tomcat add and remove threads from its internal thread pool?  I'm 
talking about the threads with names like http-nio-8080-exec-1.  It appears the 
thread pool is Tomcat's own ThreadPoolExecutor but I don't see the exact 
behavior documented.  I'm familiar with how java.util.concurrent does it, but 
it looks like Tomcat's version is a little different.

Thanks