First of all, I don't really understand about business and corporate terms. So I'm just referring it as partner/associates/alliance companies. If company A forms partnership with company B (they form such associated companies). Is there such a thing that company A poaches or quitely observe talented/potential staffs from company B in order to lure them to join them. If it does , should the B company worry about it knowing that A is 'our friend' which shouldn't be matter? Also the company A is the 'alpha' in this case.
First of all, I don't really understand about business and corporate terms.
If you are actually working in a regular company, I suggest you read up on these things. It's not that hard and very useful to understand.
Is there such a thing that company A poaches or quitely observe talented/potential staffs from company B in order to lure them to join them.
Typically not. Sometimes there is an explicitly agreement that prevents this, but it's mostly considered "bad form", i.e. there is a mutual expectation that there is no poaching. Depending on the level of people involved, poaching can put severe strain on the relationship and significantly reduces trust. In almost all cases the potential damage to the partnership outweighs the win of a single employee. So it doesn't make business sense.
I certainly had a few cases where we passed on an interesting candidate because they were working for an important partner.
That would be different for an acquisition or takeover. Poaching is a moot point since all employees are part of the new company anyway but there is certainly a good chance that some or all of the newly added employees are shuffled around in the new company.