Any economists in the house?

A viral(but not informative) Bhatnagar recently shared a graph of India, with red and blue belts. I have a quick question as to how is state wise GDP measured? Like almost all large banks are headquartered in Mumbai. So, the contributions by regional departments of those banks, would those be counted in Maharashtra's GDP or respective state's GDP? Or is this question not relevant? Or say since Maharashtra is the HQ of major banks, almost all corporate loan sanctions happen there (so that's why they have a disproportionately high share of stamp duty collection). Even if there's a manufacturing company in Jharkhand, they will have to get stamp papers in Maharashtra to get loans. Also Kerala gets disproportionate share of remittances (inwards from ME). Do these remittances get counted in Kerala's GDP. These are mostly blue collar workers, so it's not like Kerala education system is producing extraordinary people or Kerala govt incubating industries. So, that's why I am very confused how is state wise GDP computed?