Giuliani met[1] with Andriy Derkach[2] on his 2019 Ukraine trip. Derkach is the target of a long-running US counter-intelligence operation. In September 2020 the Treasury Department sanctioned[3] Derkach for his work against American interests on behalf of Russian intelligence services. Prior to the sanction, US national security adviser Robert O’Brien personally advised President Trump that Giuliani's information should be considered tainted by Russia[4].
1: Rudy Giuliani is involved
2: Rudy Giuliani specifically traveled to Ukraine in 2019 to dig up dirt on Biden, a mission that he publicly announced.
3: Rudy Giuliani is the GRU's "useful idiot" according to a report prepared for the White House by US intelligence services.