The issue is abuse of power and what would constitute such an abuse by a President of the US. It is not the exercise of a quid pro quo, as such, that constitutes the abuse of power, because that can be part of a legitimate bi-lateral trade agreement, for example, or a disarmament agreement. You give something in exchange for something. But there are times when it is clearly wrong. As follows.
The key issue relating to abuse of power is the President seeking the involvement of a foreign power in US elections. That was of course the subject of the Mueller/Russia probe - was there foreign involvement (yes) and was there complicity or collusion by the Trump administration (Mueller has ultimately left that Congress to decide). If Congress were to conclude there was collusion then that is an impeachable offence.
And that brings us to the Ukraine. In pressuring a foreign government to investigate the Bidens Trump has sought to use a foreign power to influence US presidential elections, by having them smear one of his political rivals. Inviting foreign intervention in US electoral processes is an abuse of trust in his office - and it is made worse by his doing so for personal political gain. Where the quid pro quo is illegitimate is his effectively saying to the Ukraine that military aid promised them will only be delivered in return for a public announcement from then that they would be investigating the Bidens. He is little more than a mobster making deals. He won't break their legs if they shaft a political opponent for him.