an ocean or a sea
All of the rivers in Wisconsin flow into the Gulf or the Great Lakes. One of the largest rivers in Wisconsin, that does flow into the Gulf of Mexico, is the Mississippi River.
Into ground waters and eventually the rivers and the seas.
into groundwater eventually, then into rivers and the sea!
Eventually, the rivers in New York State flow into the Atlantic Ocean. On the way there, some may flow into various sounds, bays, and harbors.
The bay that all of India's rivers flow into is the Bay of Bengal. Indian rivers are regarded sacred and have spiritual importance.
They all flow into the Pacific.
All of the rivers in Siberia tend to flow north. These rivers flow north as they being pulled to drain into the Arctic Ocean.
I would think they all flow to the Gulf of Mexico.
Downward
A river can't flow THROUGH another river but can merge with other rivers. There are some smaller rivers which flow into the River Thames including the Cherwell and Wey. The River Thames eventually flows into the North Sea east of London.
That would depend entirely on the river, but eventually it all ends up in the oceans. Some rivers flow into other rivers. Some flow into Lakes which may then feed another river or rivers. Some may even seem to just disappear (usually this would be a river that empties into a lake that then feeds off into underwater streams and rivers). The end result is the same. All water on the planet eventually makes it's way to the oceans if it doesn't evaporate into the atmosphere first.
This is an ambiguous question as there are all kinds of way we can describe river flows. For example, in the US there is what we call the "continental divide." It's located in the western US mountains. And on the eastern side of that divide all the rivers eventually empty into the Gulf of Mexico or the Atlantic Ocean. In other words, all the rivers east of the continental divide flow generally eastward. And as you might guess, all the rivers west of the divide flow westward into the Pacific Ocean. But then there are river specific flow patterns as well. They all flow downhill because their flow is based on the pull of gravity. But how they flow, fast, slowly, smoothly, rapids, eddies, and such depends on the topography that the river runs through. And man-made dams and levees can alter the natural flow of rivers as well.