Hasta La Motion!

1.I am looking for 3d multipass render footage including mattes for practise. I am learning multipass compositing. However i havent been able to come across

much footage for practise.

2. Also, I have queries about the ID mattes or the matte with RGB color. The queries are
a. why do we need them?
b. how do we use them while compositing?
c. how are they generated?

If you have any link to the footage or footage of your own kindly help me. It is purely for practise purpose.
Any link to tutorial or knowledge resource about ID mattes are also welcomed.

Thanks & Regards
Jetin Anand