Defined benefit means that the employer buys a perpetuity from the insurer on behalf of the pensioner. The insurer requires the employer to pay the cost of the benefit divided by the average rate of return after operating and profit margins.
If the insurer can earn 6%, and takes 1% of that, then if an employer wants to offer an employee a $50k/year pension, they need to pay in $1M before the employee's retirement to fully fund it. Over an expected 40-year career, if that employee was basically making $50k/year in take-home pay the whole time, adjusted for inflation, the pension cost would add $25k/year.
Actually, the cost to fund it would be a bit less, as the pension usually ends when the pensioner and their spouse die, so not a true perpetuity. The insurer calculates the expected time from retirement to death, and discounts the up-front cost by the current value of all those payments that won't be made after the beneficiaries die. This works out better the more pensioners can be averaged out in a big pool, which is why big insurers have an advantage in offering these sorts of financial products.
But no matter how you slice it, defined-benefit plans, if fully funded by incremental contributions with each paycheck, add significantly to labor costs, especially when the insurers can't get good investment returns. The lower the return, the more the plan costs. And the higher the return, the more the insurer tends to take for itself.
The 401(k) reduces the pension cost to the employer by capping it at a lower percentage of employee take-home pay, and furthermore dumping all the market risk onto them.
Completely different. Defined benefit has always been better for the laborer, insofar as the company can keep its promises. Defined contribution is better in cases where the employer is untrustworthy or the employee can invest wisely such that they can get the same returns as the insurer, but without taking a cut off the top--as one might get with no-fee, broad-market index funds.