1. The FBI and other U.S. intelligence agencies can and do operate overseas, under certain circumstances.
2. Hunter Biden is a citizen of the U.S., and these agencies can detain and question him on U.S. soil if warranted.
3. Or, you know, the Ukrainians themselves can decide whether or not to investigate a certain individual for corruption, free of undue pressure from the U.S. president.
Look, Hunter Biden does seem to me like a petty grifter and beneficiary of Biden family nepotism -- not trying to defend everything he's ever done. But he's clearly not some sort of imminent national security threat that would justify the personal involvement of the commander-in-chief.