Learn English – Meaning of the following sentence

I would have been able to comment had this still been in EL&U , where it should have remained because it's an interesting grammatical question, so you'll have to settle for a short answer.

Although "what have you been doing" may have a form whose name includes "perfect", the statement doesn't have a perfective aspect - because the questioner hasn't decided the person has finished doing whatever it was.

Wikipedia says

Perfective aspect is used in referring to an event conceived as bounded and unitary, without reference to any flow of time during ("I helped him"). Imperfective aspect is used for situations conceived as existing continuously or repetitively as time flows ("I was helping him"; "I used to help people").

Had the questioner concluded the activity was over, they would have used the perfective "What have you done?" Also be aware a statement like this can imply something bad was done, so the questioner might back off from the perfective to avoid sounding rude.

Some languages (e.g Polish) have completely different verbs to express perfective or imperfective aspect; in English we settle for progressive/continuous tone.

Your simple past understanding is fine. In your example the present perfect and the present perfect continuous are practically interchangeable. In both cases, David is still working for them. With 'how long' questions it is often the case that the two are interchangeable.

Sometimes there is a slight difference between the meaning of the present perfect simple and present perfect continuous. They both have an effect that is valid in the present, but sometimes in the present perfect the action is complete while in the present perfect continuous it doesn't have to be.

A typical grammar book example is 'I have painted the ceiling' versus 'I have been painting the ceiling'. In the first example, the effect is that I am happy now, in the present; the job is done. In the second, the emphasis is on the process, the action. We do not know if the job is finished, just that the speaker was recently involved in painting the ceiling.

PS I should add that this is British English.