Giuliani has ties to corrupt elements in and out of Ukraine's government and was pushing long before 2019 for the government to manufacture scandals against Democrats, back in 2016-2017 against Hillary Clinton mainly.
Biden's name first shows up on this one in late 2018 (as Trump's already concerned about the 2020 election). Second mention is March 2019, when Ukraine obliges by opening investigations.
Late 2018 Two Soviet-born Florida businessmen, Lev Parnas and Igor Fruman, arrange a Skype call between Giuliani and Shokin, according to an investigation by the nonprofit Organized Crime and Corruption Reporting Project (OCCRP) published in BuzzFeed News. The two businessmen also connect Giuliani with then-Prosecutor General Lutsenko. Giuliani invites Lutsenko to his office in New York, a meeting they arrange for January.
Lutsenko told the LA Times that he had numerous conversations with Guiliani on the phone. He also tells the paper that Giuliani pressed him repeatedly to open an investigation on the Bidens and Burisma, even though Lutsenko had seen no evidence of legal wrongdoing.
March 2019 Ukraine prosecutor general Lutsenko opens two investigationsone into the 2016 U.S. presidential election and a second into Burisma and Biden.
Following his meetings with Giuliani, prosecutor general Yuriy Lutsenko announces he is opening two investigations.
The decision to reopen the investigation into Burisma was made
by the current Ukrainian prosecutor general [Lutsenko], who had cleared Hunter Bidens employer more than two years ago. The announcement
was seen in some quarters as an effort by the prosecutor general, Yuriy Lutsenko, to curry favor from the Trump administration for his boss and ally, the incumbent president, the New York Times reported (in May 2019).
https://www.justsecurity.org/66271/timeline-trump-giuliani-bidens-and-ukrainegate/