Giants are found in many different Mythologies. None of it makes much sense.
Some people still believe these myths
Hindu Mythology called giants Daitya. Some fought the gods as in Norse mythology but others were friendly giants.
In the bible, giants were born after the children of god (the Demons) had sex with the daughters of men. (Genisis 6:4) ”There were giants in the earth in those days; and also after that, when the sons of God came in unto the daughters of men.” They were violent brutes. One famous giant was Goliath. He was also violent and strong, but somehow he was killed when a kid threw a rock at him.
People don't believe these myths any more
The Greeks, Romans. Celts and Nordics had ideas about giants as did many other peoples. In Greek mythology, there were giants known as Titans, which were god-like. They were created when Cronos defeated Uranus. Other Greek giants include the Cyclopes, which had one eyeball. In Norse Mythology the giants spent quite a lot of time fighting the gods. The whole Earth was made from the flesh of the giant Ymir. The Celts had a story about Bran the Blessed.
People believe this
There are hoaxes of really enormous giants from time to time, see "Skeleton of Giant" Is Internet Photo Hoax as an example.