Of the 15 or so Indian cookery books I have this is my favourite.
https://www.amazon.co.uk/Curry-Secret-Indian-Restaurant-Meals/dp/0716021919
It explains that almost all Curry restaurants use a same basic sauce. Onions/Ginger/Garlic in a slack paste.
They keep cauldrons of this ready prepared paste and when the orders come in from the covers the chefs just add whatever extra herbs/spices/flavourings to make a Korma/Vindaloo/Phal whatever.
This book is described as the book 'restaurant owners don't want you to see'
because it is so like the recipes you expect from a restaurant cooked in your own home.
As I type I am cooking a chicken curry which I found yesterday in the BBC Good Food section which uses the holy trinity paste I described earlier plus cumin/fennel/tumeric/chilli flakes and Garam Masala etc.