what is anatta during vipassana practice?

When in meditation observe fabrications that arise in your mind. If you see that no matter what fabrication you observe, the observed fabrication is not you nor it's yours, that is the experience of anatta.

You said that during your meditation mind itself can be seen. Do it again and ask yourself if this mind you see is you and is it yours? If you clearly see that it is not you nor it's yours, that is the experience of anatta.

Next, experience anatta by observing as much fabrications as possible in your mind.

Next, experience anatta by observing as much fabrications as possible outside your mind, in the world/universe.

No matter where you'll observe, inside or outside, you'll experience anatta.

Na matter where you'll search, you'll experience anatta.

If at the end of your search for self (atta), you experience anatta and just anatta (no self), then you'll know anatta: all is not self. All is empty of self.

This is knowing anatta by experience.

Once anatta is known in the way I described, then the realization of the dhamma is not far away.

During vipassana meditation mind experiences breathe automatically. no thoughts were coming. mind itself can be seen.is this anatta?

Seeing anatta is seeing the natural absence of 'self' in things; such as the absence of self in a drop of water.

If the mind can see its knowing & consciousness (awareness) is without self; if the mind can see it is the physical body that breathes rather than a 'self' that breathes; if the mind can see the breathing is merely 'air' or 'wind' (rather than 'my breathing'); and if the mind can see the automatic happenings in meditation occur without any self; this is beginning to see anatta.

What you are describing is a state you reach when you practice meditation. This cannot be anatta since anatta is not a state, it is a property of existence present in all things.

Rather, your description looks like you have reached a state called access concentration. This is the first but important step in practice. This is the state where mind no longer gets lost in usual wanderings, and breathing becomes relaxed, with mind staying with the object of meditation (breathing). Thoughts don't arise or if they come, they stay in the background, because the subtle tension that gives rise to thoughts doesn't become strong enough to distract and goes to the background (This is what I believe you meant by mind itself can be seen).

Also, your description that mind itself can be seen also tells me that is not related to anatta, as that itself is a thought (without that you would not know that it is mind you are seeing), which means you are in the identification stage and not (yet) seeing the true nature of things. Observing anatta is seeing the grasping/identification activity in action and noting that that is not me (if it is, then who is the I that is watching it? - hence a contradiction so both are not me in essence).

I don´t know. If you look inside whilst thinking, ask yourself "What is this?" Try to find the one who is thinking. Then the thought just vanishes, because it is Not-Self, not you nor anything else, but a changing phenomenon that comes and goes. Just let it go by asking What is This? There is no answer in your brain, you just need to see that there is nothing behind a thought but emptiness (no entity)

No. Anatta arises from seeing the arising and passing nature of conditioned phenomena wherever you look. You see that the whole body is like mass of bubbling particles. One set gets destroyed another arises. You also see that this process is not controllable and this process generates sensations which are essentially unsatisfactory. Unpleasant sensations are unsatisfactory, pleasant sensation are unsatisfactory when they end and neutral sensations are unsatisfactory as they are conditioned and pass away giving rise to unsatisfactory experiences.

In addition, craving towards any of the sensation gives rise to the notion or perception of a being. Such perceptions all end in unsatisfactoriness.

Whatever unsatisfactory is not worth identifying as self. This is Anatta.