In simple terms, GST is a tax applied on incomes of non-government, private entities, so it is applied on Exchange Turnover Charges (which is Exchanges' income), Clearing Charges (income for the clearing-houses) & brokerage (obviously, income to the brokers); rest of the stuff like STT, Stamp Duty, SEBI Charges, etc. are taxes themselves, so no GST on them.
Often, Exchange Turnover Charges & Clearing Charges are clubbed together as one Transaction Charges since they relate to the trade transactions. Some brokers, like, Finvasia, IB & others clear their own trades, so they don't have to pay Clearing Charges to someone else, so they don't levy those charges on their customers whereas brokers that don't clear their own trades, have to pay clearing charges to a clearing house to clear their trades, & obviously, those brokers pass those charges on to their customers.
Edit : Forgot to mention that, YES, two brokers with the same brokerage-structure can end up with different GST amounts, given that there might be differences in Clearing Charges charged by each one of them.
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