This study is researching experimental drugs called REGN7508 and REGN9933. The study is focused on participants who have atrial fibrillation, which means that the heart beats too fast and unevenly. REGN7508 and REGN9933 are designed to help stop blood clots forming in patients with atrial fibrillation. The aim of the study is to see how well REGN7508 and REGN9933 work in patients that get medicine for their atrial fibrillation. The bleeding effects of REGN7508 and REGN9933 will be compared to another medicine (apixaban), which is available on the market to treat and prevent formation of blood clots. The study is looking at several other research questions, including: * What side effects may happen from taking REGN7508 or REGN9933 * How well do the study drugs reduce the risk of having a stroke * How much of REGN7508 or REGN9933 is in the blood at different times * Whether the body makes antibodies against REGN7508 or REGN9933 (which could make the drugs less effective or could lead to side effects)