Are capital gains tax based on total investment or the withdrawn amount?

Sorry if this question has been asked before, I tried searching for an answer but maybe I am not phrasing it the best that I can. So say I had 500k invested into an index fund and it was growing at 7% annually. If I take 3.5% of my earnings would I be paying taxes on the full gain of $35000 or only the amount I took out which is $17500? And if the former is the case how would it work if I'm constantly doing this every month/year?