Technically speaking details are not announced yet, though the popular assumption is that this will be a paid service not unlike what Boomerang launched this year. It will launch at some point in 2018, Young Justice S3 and the live-action "Titans" are the original projects confirmed for it. That's what came from the initial announcement, and all else counts as speculation. However, let's be serious. DC animation expiring on Netflix isn't arbitrary. They need content to draw eyeballs to this enterprise, and it really doesn't make sense to have two seasons of Young Justice somewhere else while season 3 is by itself. Netflix was important to S3 coming back, but what they've gained from having YJ there is finished now. I don't see how this service could be profitable on advertising alone, frankly. I expect a paywall. It's not impossible that at least some things will be free to view, or that episodes of Young Justice will be viewable with ads and / or made publicly available after a given period where they are only for subscribers.