All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
A Fatwa is not enough to explain and detail what you have asked about; rather, this needs a complete book.
Nonetheless, what we can say in brief is that there are many reasons for the emergence of the different sects and the differences amongst them. Amongst these reasons are ignorance of the revelation that Allah revealed, injustice and following one’s whims, following the rule of the majority and blind imitation of others, envy, dispraised competing in worldly matters, abandonment of enjoining good and forbidding evil, basing loyalty and allegiance on tribalism or ethnicity, and other matters of the Jaahiliyyah (Pre-Islamic Era of Ignorance). An additional reason to these is the plot of the enemy against the Muslims (to cause disunity). Each of these reasons contributed to the emergence of the division, differences, and the enmity amongst these sects.
Allah knows best.