I had hoped to quit my job at 66 mostly because I don't like it any more, and not collect my SSI until I am closer to 70. However, when I plugged some numbers into the SSA's calculator it shows my benefit dropping if my salary drops. What happens if I take money from my 401K for a few years? Will my SSI drop instead of increase? I'm wondering where I can get some clarification on this?
I'm thinking you mean you want to collect Social Security retirement ("old age") benefits. "SSI" is an entirely different program although, for the convenience of the federal goverment, it is managed by the Social Security Administration. SSI is Supplementary Security Income and is essentially "welfare". See https://www.ssa.gov/ssi/
(1) Your old age benefit is based on the highest 35 years' worth of income; if there aren't 35 years of wages then the empty years count as "0". So it is possible that someone could increase their benefit by working beyond their full retirement age to increase the "high wage" years. This may be your situation but I suspect it is unlikely.
I'm also assuming that you are of such an age that you have those 35 years of wages. If you don't then the online SS calculators may not give you good results.
Are you registered for a "My Social Security" account at the SSA? This will give you an estimated benefit amount for your full retirement age and for drawing sooner. If you are younger than your FRA and still working it assumes that you will make wages in-line with your recent history. So that may be inaccurate for some situations.
(2) You can draw money from your 401(K) or IRA, without penalty if you are of sufficient age. And this will not penalize your SS old-age benefit. If you really are speaking of SSI then receiving such income from retirement accounts may very well reduce a SSI benefit. (remember, SSI and Social Security are entirely different programs.)
(3) To get further clarification, you can call the SSA to discuss. You might want to further clarify your situation and ask questions in this forum. And you might use Google or other search engine to find some information sites. Even AARP has a lot of information on Social Security. https://www.aarp.org/retirement/social-security/