We don't really know yet but it is supposed to be in the Arrowverse
but like a different Earth? Because isn't there already some kind of JSA/Stargirl in the Arrowverse?
You don't think they have the budget to pay for Jack? They have to pay Johns for Stargirl... oh, wait.
You wouldn't want to "waste" Jack on a CW or even a DC Universe (RIP) show, though, you'd want the full HBO/HBO Max treatment.