When President-elect Joseph R. Biden Jr. moves into the White House, he will inherit a relationship with China that is at its lowest point in decades. Among the issues on his to-do list are tariffs, which he has said he plans to maintain when he takes over; how to handle human rights violations in China’s Xinjiang region; and a tech Cold War that has raised questions over data, sovereignty and power. All the while, Mr. Biden will be rebuilding an economy laid low by the pandemic. That could make focusing on foreign matters more fraught, especially a relationship in need of as much repair work as the one with China.