Is Stargirl in the Arrowverse?

And if so, was the Golden Age Flash on the JSA? Just wondering as I’m watching The Flash and Stargirl right now.

Yes and yes

Not in the same universe, but, post “Crisis,” in the same multiverse. Stargirl takes place on Earth-2 to the Arrowverse’s Earth-1. The Earth-2 we saw in earlier seasons of Flash and Arrow has been rewritten as Stargirl’s world.