Biden was threatening to uphold the agreement that had been previously laid out between the US and Ukraine. It's not a QPQ if it was already agreed on. Trump wanted Ukraine to investigate someone specifically. Someone who was a political rival. The agreement was to investigate and prosecute corruption in Ukraine. The investigation involving Hunter did show that Hunter joined Burisma after the time of the alleged corruptive acts took place, making him innocent of all claims. Yet, Trump pushed the agenda anyway.
The US was not the only one who had such an agreement with Ukraine either.