There always has to be a mad scientist. Dr. Wilfred Nagel is introduced in Episode 3, and his history is almost spot on with the comics. In the show, Nagel worked for Hydra before the CIA hired him. He used Isaiah Bradley's blood, given to him by the US government, to re-create the Super Soldier serum. In the comics, Dr. Nagel was the villain who experimented on Black soldiers, including Isaiah Bradley, so the show isn't too far off here.
The Falcon and the Winter Soldier revealed that Dr. Nagel disappeared because of the Blip. When he returned, he found his work abandoned and the project in ruins. However, the Power Broker decided to fund his experiment, giving it new life. And that's how we have the new Super Soldier serum.