It's hard to nail down exactly what claims he's making, because he (and Giuliani) often just rattle off numbers and soundbites without providing any actual details, dates, or evidence. I suppose that's par for the course in a Presidential debate and both candidates kinda did it yesterday. I think you might be mixing up two separate accusations Trump & Giuliani have made: the first is that Biden received 1.5 billion from China, and the second is that he received 3.5 million from Russia.
Both of the claims are focused on money received by an investment group called Rosemont Seneca Thornton (I
hate investment company names). RST was formed of two investment firms, 'Rosemont Seneca' and 'Thornton'; Hunter Biden was a co-founder of 'Rosemont Seneca'.
In the case of the 3.5 million from Russia, that's based on a "consultancy fee" of 3.5 million dug up by Senate Republicans which was supposedly paid by a Russian businesswoman (and the ex-Mayor of Moscow's widow) to RST. The report the Senators compiled didn't provide any detail beyond that it happened.
The 1.5 billion is a bit sillier. In 2013, RST combined with a Chinese "equity fund", Bohai Capital, and formed
BHR. Hunter Biden (as co-founder of one of the three firms that make up BHR) was named to its Board of Directors.
The 1.5 billion comes from a
fundraising target they posted; it's not actually money given to Hunter, or Joe, or anyone else.
I hope you appreciate this, I gave myself a minor headache trying to untangle this finance-world nonsense.