Where do passengers of refuelling stay?

Technical stops just for refueling are no longer particularly common in commercial aviation in most of the world. Long-range aircraft often make them unnecessary, and full on ordinary stops and, where legally possible, connections provide more flexibility for airlines and passengers.

Many are unscheduled (planned, but not routinely part of flight schedules) stops due to high winds. A number of transatlantic 757 flights in 2012 needed to make such stops, and it can occasionally happen on unfavorable weather on transpacific routes too. In such cases, passengers generally remain on board the aircraft while fueling takes place; no visas are required. If passengers are allowed off the plane, they won't pass through immigration controls.

However, there are some regularly scheduled stops that work differently. These stops do involve taking on required fuel, but serve other purposes. British Airways operates a flagship service from London City Ariport to JFK with an all business class A318 aircraft. Due to LCY's short runway, the aircraft takes off from London with limited fuel, stops at Shannon, discharges all its passengers so they can go through US immigration preclearance, fuels up, boards, and completes its journey to New York (you can read about that process in a review). Kuwait Airways makes a stop at Shannon between Kuwait and New York. Passengers get off the plane and clear security (but do not go through US preclerance or any other immigration control) and wait in the departure lounge before re-boarding (after fuel and a fresh crew). Qantas operates an LAX-JFK flight to provide onward connections for their flights from Australia (they do not have the cabotage rights to sell tickets on this flight to solely domestic passengers).

There are a few other examples of such flights. Whether or not a visa is required for these scheduled stops depends on the exact circumstances of the flight, the country's immigration rules, the passenger's nationality, and sometimes whether they already possess other visas, so that's a specific thing that would have to be researched for a given flight.

In summary: passengers on unexpected fuel stops generally stay on board or near the gate, while the experience for passengers on regularly scheduled stops will vary.

I'm not aware of any flights to the US that routinely make a fuel stop in China. If a stop were necessary in an emergency, provisions can be made to keep passengers in the terminal building. In case of long emergency stays, countries can and often do waive visa requirements in emergency situations, sometimes holding passports while passengers go to a hotel (see, for instance, the US rules authorizing waiver of the visa requirement in case of an "unforeseen emergency").

Umm they stay on the airplane.

It depends on the flight and local procedures.

In general the passengers are disembarked (see it as a two connecting flights).

Sometime passengers could remain on the plane (but usually when most of people disembark. Refueling is considered a dangerous activity, so there are various special procedures and how many flight attendants should be ready (and where). On pilot should check from outside. In any case the airplane should be ready to evacuate. As far I know, because time constrain, it is not possible to have a refueling plane full of passengers.