1) Could you please help me with the difference between rabbul alameen and maula e kainat?
2) Also, why do we call Imam Ali (as) ‘Maula e Kainat’. It will be of great help if you could educate me with some authentic hadith.

The difference between God and Imam Ali is that God is God and Imam Ali is the slave and servant of God. One is the worshipped the other is the worshipper.

Imam Ali is called that because he is the leader after the prophet. He is the leader of all.