This is also a nested statement, one statement in a the body of the fork, and the fork counts as a statement. Statements are made up of commands or expressions. Commands tell the moo to do something. Commands do not return values. Commands can take expressions as arguments, but commands can't be used as arguments.
Expressions return a value.
It returns 0. In this case, the expression is 1, which returns 1. While does not return anything. It's a syntax error to assign the result of any command to anything.