You don't pay capital gains. Capital gain is the profit that you make when you sell an investment for more than you paid for it. Capital gain is income to you.
You pay income tax on your ordinary income, and you pay income tax on your capital gains. The income tax rate that you pay on capital gains is lower than the rate you pay on ordinary income. The tax rates for ordinary income, such as the 12% rate that you mentioned, do not apply to capital gains. There is a separate schedule of rates for capital gains, with different income cutoffs and different tax rates.
The tax rate on capital gains depends on your total taxable income (not AGI), including the capital gains.
It's not clear what the significance of "74K" is. That's not the cutoff for any tax bracket for 2019.
Here's an example of how the tax on capital gains is calculated. Suppose your filing status is married filing jointly, your taxable ordinary income is $70,000 and you have capital gains of $20,000. Your ordinary taxable income of $70,000 will be taxed partly at 10% and partly at 12%. The $70,000 of ordinary income uses up $70,000 of the 0% capital gain bracket. For married filing jointly, the top of the 0% bracket for capital gains for 2019 is $78,750. So the first $8,750 of capital gain will fall in the 0% capital gain bracket and will be taxed at 0%. The $8,750 is the top of the 0% bracket, $78,750, minus the $70,000 of taxable ordinary income. That uses up the 0% bracket. The remaining $11,250 of capital gain will fall in the 15% bracket and will be taxed at 15%.
You can find a chart of the long-term capital gain tax rates for 2019 at the following link. At that link, scroll down a bit to see the chart.
Long-Term Capital Gains Tax Rates in 2019
