US players can play in both state-based and offshore online casinos. Players are unrestricted in their choice of casinos, unless a website uses specific geolocalization software. For example, in the United States, players in New Jersey cannot play at online casinos in Nevada or Pennsylvania, because of that software.
However, when you pick offshore online casinos, players from all states are permitted to participate. Some minor exceptions per casino exist, but for this, you will need to look up the T&C’s of each specific brand.