How did React Fiber optimize reconciliation

I have read few articles where with React Fiber and here are some questions.

1. with previous stack reconciliation why is parent-first, depth-first-search impossible? I saw posts saying React Fiber is implemented with linked-list enabling those features. but post also mentions that stack reconciliation works "recursively". what is the difference?

2. React Fiber keeps current tree and workInProgress tree. One of the key features of React Fiber was that it doesn't recreate React Elements and only updates(mutates) changes. does this mean that current tree and workInProgress tree share(reference) same elements or does it clone?

3. If React Fiber can resume and pause, does this mean that it keeps track of current progress and simply stops when 16ms deadline comes and commits whatever is done? and continue where ever it left off?

4. Before React Fiber, elements were immutable. Does this mean that if React found a element that has to be modified, all of its children elements were re-created and attached to the newly created element?