It seems to have been an investment company that invested in gas and oil. Burisma - of which this is sort of Act 2 - actually was involved in controlling gas and oil pipelines and sales, which were a point of conflict between Ukraine and Russia (as well as Romania and just about every other country in the region).
During the Burisma episode, it looks as if all of our foreign policy was based on promoting Hunter’s attempts to get control of different gas pipelines, working through the corrupt leaders of countries in Eastern Europe, the Orient and even Latin America. He knew nothing about the business, but was good at getting his company front and center for any benefits from his connections, which was why he was valuable to them.
He then segued into the Chinese stuff, and that was when Bobulinski, a legitimate businessman who had founded and run investment companies, was brought in as a sort of front man. Biden wasn’t in office at that point, but it was shortly after he left and obviously it didn’t happen overnight but had been in the works while Biden was still VP.
My point is, I am positive now that Trump and Zelenskyy exchanged a remark or two about the previous administration. Four days later, FOUR DAYS LATER Biden announces his candidacy for President. And of course, once he was a candidate, investigating the previous administration was "election interference" and "an impeachable offense."
I think Biden is running mostly to prevent an investigation into the Obama/Biden administration. They were undoubtedly hoping/expecting to impeach Trump, after which Biden could drop out, but that didn't work. So now he's running out the clock. He may actually hope he can win, but even if he can't win, as long as he's running, Trump can't investigate.
This also means that if Trump was already alert to the corruption in April, he took pains not to have that awareness reflected in the phone call, but someone in his administration warned Biden.