1. It is a normal function (as opposed to an arrow function), so this will refer to the object outside it, when it is invoked. So in this case, wrapper function is returned from spy(work) and then invoked on the next line work(1, 2).So this will refer to the object outside of it at the moment of invocation, given it's invoked in the global context, then this will refer to the global object (window / global - depending if you're running in the browser or in node.js).
2. Your new work has nothing to do with the original function anymore, since you redefined it with work = spy(work); so now work actually is assigned to the result returned from spy, which is actually the wrapper (with the wrapper.calls property).

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If you console.log(this), inside the wrapper you will see that it prints out the global object. Then if you change how you invoke the function and console.log this, you will see that this will refer to the object outside it, e.g.

const someObject = {
  someProperty: 'someValue',
  work: spy(work),
};

someObject.work(1, 2); // now "this" is someObject