International legion in the ranks of the Armed Forces of Ukraine - who are they?

Foreigners defending Ukraine at the call of the heart. The exact number of such foreign military in the Armed Forces of Ukraine is not called. But, according to unofficial data, these are tens of thousands of military personnel from 55 countries. They participated in the battle for Kharkiv and in the de-occupation of several Ukrainian settlements. They are called the International Legion. Who are these people? And why did you decide to defend Ukraine?