There's almost zero detail about the broader story of GTA VI, other than Lucia being in prison, presumably at the start of the game. Other highlights of the trailer include Florida's swampy Everglades National Park, an airboat, some wildlife and, of course, a strip club. It also appears that the game will have a playable female character, Lucia, for the first time in the modern incarnation of the franchise, just as the rumors predicted.
It remains to be seen if those streams are an integral game mechanic, purely narrative tool or just an aesthetic choice for the trailer, though. Given that the trailer features a ton of Instagram-style live streaming, GTA VI seems to be a contemporary game, rather than one set in the '80s like 2002's Grand Theft Auto: Vice City. As indicated by a recent teaser image, leaks and various rumors, GTA VI will be set in Leonida, Rockstar's take on Florida, and largely centered on Vice City, the series' stand in for Miami.