Learn English – “I have been doing this job” vs “since I am doing this job”

It is "have been". It is basically never correct to say "am been", and it wouldn't mean what you want it to mean anyway.

One possible, correct sentence is:

I have been, for a short time, an employee of that company.

Or (this is more natural for me, but arguably less correct):

I have, for a short time, been an employee of that company.

Either way, it means the same. It says that you were employed a short time ago. It doesn't mean you're about to leave. I see why you might want "from" in this case, and not "for", but don't panic: in this case, "for" does not mean that it will only be "for a short time"! Just that it is "a short time" so far.

Now, you said you didn't want a grammatical or technical explanation, so we can stop there. But in case anyone else wants to know the grammar behind it, I'd like to elaborate a bit anyway.

A construction like "have been" is called the present perfect. It is for actions that are past and finished, at this moment. Its structure is have + past participle.

(There is also a past perfect, which is for actions that were already past and finished at some previous time. Its structure is had + past participle.)

A construction like "am being" is called the present continuous. It is for actions that are happening (right now), or that often happen (not necessarily right now, but before now and, we expect, after now too). Its structure is be + present participle.

But "to be" leads a complex existence. It is also used in ways that are basically unrelated to the present continuous.

A construction like "am been" is called the present passive. It is for actions where the subject ("I", in "I am been") is the recipient or target of the action. Its structure is be + past participle. You might say "I am insulted" (someone gave you an insult), or "I am seen" (someone saw you), or "I am blessed" (someone gave you a blessing).

It would be very unusual to say "I am been", though. That would mean someone was being you. If an English speaker really wanted to say that, they would almost certainly use different words.

Out of your 4 sentences, three are correct. The only one that is not correct is:

Until you called me, I'd visited a store and bought some foods.

Until is used for to specify a duration going on up to the point specified by what follows until.

In the following sentence, the action of studying for about 3 hours had been going on up to the point where the speakers was called.

Until you called me, I had been studying for about 3 hours.

The same sentence, but with before, is also correct, but there is a difference in meaning. The studying here also took about 3 hours, but it had already stopped at the moment that the speaker was called. So, the caller did not interrupt or stop the person studying:

Before you called me, I had been studying for about 3 hours.

The third correct sentence is:

Before you called me, I'd visited a store and bought some foods.

The action was completed before the speaker was called. This is the reason that until cannot be used here, because that would mean the action had been going on up to the point where the person was called. The past perfect had visited here implies that the speaker had already finished the action of going to the store to buy food.