If it's India specific then you can register for GST once you cross 20 lacs in turnover.
No company deducts GST while paying you. They can deduct TDS though. I'm not aware of Amazon India's payment policies.
I'm not sure what do you mean by your last question.
I haven't started making any money yet, and I doubt I will ever reach 20L from affiliate earnings. So in that no GST registration right?
About the last question, if I do register for GST, does that mean I have to buy subscription of certain services (Ahrefs, SEMRush, KWFinder, etc), or can I still buy group buy services for cheap? Per my cursory reading, it seems if I do need to buy full version for these services, then there is something called Reverse Charge Mechanism under GST that has added compliance?
Sorry if I'm not making any sense.