Assuming he retains control of the Senate, McConnell can obstruct any new legislation he wants, but Trump did not actually pass a lot of laws. He did almost everything by Executive Orders and temporary appointments. Biden can undo those on Day One. So we would be right back to the situation under Obama.
Then Biden can appoint qualified people to head departments, and for cabinet positions. McConnell could refuse to confirm Biden's cabinet nominees, but Biden would have scores of good people for each post. McConnell would start to look more and more like an a-hole, trying to sabotage the American government.
Then Biden could issue as many Executive Orders as he wanted. He will actually have very competent lawyers who can craft them to be constitutional, so they have little chance of being rejected by the courts. His people at the White House will actually follow the correct procedures, so the courts will not be able to fault the process, as was so common for Trump's orders.
So the Biden term in office will be very much like Obama's. Apparently, voters just wanted to get rid of Trump, but not for Biden to pass laws that change things a lot.