How were there giants in the land of Israel when the Jewish people arrived? As far as I know, Og was the only giant that survived the flood. Were all these giants descendants of him? And did they only live in Israel?
The Da'as Z'keinim Bamidbar 21:34 writes that Sichon's mother was impregnated with Sichon by one of the giants who lived before the flood, and married one of Noach's sons and entered the Ark while pregnant with Sichon. He was then born on the Ark. So Sichon is a second possible line of lineage for the giants to survive.
Bereishis Rabba 33:11 records the opinion of R' Yochanan that the flood did not affect Israel. Therefore, all the giants living there could have survived, and that's why they found them there.