What triggered the start of the Trojan War? And what is a Trojan horse?

If you believe the ancient Greek epic, then the reason for the beginning of the Trojan War was the abduction of Helena the Beautiful by Paris. She was the wife of the Spartan king Menelaus. While sailing, she did not forget to take with her the treasures of the Spartan king. On demands to return Elena, Menelaus was refused and began to prepare for war.
The Trojan Horse is an epic horse-shaped construction that the cunning Greeks donated to the Trojans. Greek soldiers were inside the horse. The inhabitants of Troy dragged the horse into the city. At night, the Greeks got out of the horse and opened the gates of the city. The rest of the Greeks penetrated them. Troy fell. There is a version that the Trojans had to disassemble the gate because the horse did not fit the size.