Doubt regarding the indian oil and petroleum industry

So, i don't know much about this industry, i am confused as to what does IOC, bpcl, hpcl do, where they get the petrol diesel from, do they extract crude oil themselves, as that would be highly volatile, since crude prices gets changed every day, yet the petrol diesel price stays constant at the pumps, so what does ongc, oil india or reliance do, i know about reliance that it refines the crude oil, so does that means ongc extracts crude oil, what price play happens Like it is clearly visible i am very much confused, please help me find resource to understand all this