I'm still not sure if you understand or not. So let's go through some examples. Please note these numbers are completely arbitrary, and are simply used to illustrate the mechanics of the credit.
So, let's say you make $100,000 during the year. In scenario A, you don't have any withholding during the year. Your marginal tax rate somehow comes out to be 25%. In April of the following year, you have to write the IRS a check for $25,000 (actually, it would be more, cause the IRS gets mad and charges penalties if you owe too much at the end of the year... except, except, except...). If you had purchased a Tesla in that year, you would be eligible for the $7,500 credit, so you would only owe the IRS $17,500.
Now for the scenario you seem to be confused on. You made the same amount of money, but this time, you had $1000 withheld from every bi-monthly paycheck. Come April, with no EV tax credit, you would owe the IRS an additional $1000. If you had bought a Tesla in that year, you would still be eligible for a $7,500 tax credit ($7,500 is less than your total tax liability of $25,000). So you would get a refund check for $6,500, instead of owing $1,000.
For the "non-refundable" part to kick in, your tax liability would have to be less. Say you made $100,000, but $80,000 of it was from a tax-free Roth IRA. Your AGI would be $20,000 (not really, due to exemptions and such, but this is where I'm simplifying), and if you somehow had the same 25% marginal tax rate, your tax due (assuming nothing withheld during the year) would be $5,000. With the $7,500 tax credit, it would reduce your tax due to $0, and you would owe nothing, but you also wouldn't be able to get a "refund" beyond how much ($0) you paid in. Given the same scenario, and you were a responsible individual that had made estimated tax payments of $5,000 throughout the year, your tax due in April (without the EV credit) would be $0. With the EV credit, you would be due a $5,000 refund - the amount that you had paid in, and not more.
To address your specific quote of "incentives/credits being the LAST factor" is your error. Credits are a higher priority than your own payments. Regardless of temporal order (estimated tax payments and payroll withholdings happen earlier in time than claiming the tax credit), you still get to benefit from the tax credit, even if you're due a refund already.
Also remember that there are as many tax situations as there are people in the US. Just cause your friend was not able to claim the credit, even though they apparently make a lot of money, doesn't mean that you won't be able to (or vice versa). It really depends on how you make your money. The tax code is a super complex beast, with a million different credits, exemptions, and deductions, which all work a little differently (not to mention AMT, NIT, and any other way the government can come up with to still get their money).
