question on selling stocks/etf/indexes and what gets taxed when and how

I've not really been able to find a concise answer; hoping someone can help a layman out... Most of my money is in retirement (although same questions apply eventually), but I've started dabbling with Ameritrade the last couple years...

I understand the basics of paying taxes only on your profit, and the short term and long term capital gains. My question is, is there a standard on how this is done, and who has to track/know it?

That can be a confusing question, so let me explain with an example:

Let's say I buy 10 shares of VTI 1.5 years ago (A). And I buy 10 shares of VTI 0.5 years ago (B). And today I sell 12 shares.

1. Which 12 shares? Is it FIFO - 10 from A and 2 from B? Or is it 2 from A and 10 from B? Is it 6 and 6? :)
   
2. And, I would ASSume... is the profit basis directly tied to each of those based on cost basis, e.g. cost/profit for each share based on whether it's A or B, or is there some averaging that occurs?
3. Is there a standard that any brokerage would follow, or are the answers to the above "it depends on the brokerage."
4. Who generally has to track all this? In other words, I've gotten 1099/1098 forms in the past - would that have all the info from the brokerage that I'd report to the IRS, or do I need to be figuring out the above amounts?

I would like to better understand the process (questions 1-3) but also want to learn my own responsibilities (4) or just trust the brokerage (or retirement manager). Thanks!