Hi, it says that Og survived the flood by latching on to the roof of Noah’s arc while being fed by Noah, so my question is where did Sihon come from? How did he survive the flood?

ב"ה Shalom It is more commonly mentioned in the Rabbinic literature that Og was saved from the flood, though in a manner different than the one mentioned. However, there are midrashim and the Chizkuni which say that Sichon was saved as well. (דברים רבה (וילנא) פרשת וזאת הברכה פרשה יא, חזקוני בראשית פרק ו ,זבחים קיג ע"ב) All the Best