Backdoor Roth IRAs involve making a traditional IRA contribution and not taking the deduction at tax filing time (because you can't), but then rolling over (not recharacterizing, that's something else) the traditional IRA contribution into a Roth IRA. It's completely tax free, assuming you have a $0 traditional IRA balance once the rollover is complete. The usual way to accomplish this is to roll all traditional deductible IRA balances into a traditional 401k first.

What you're kind of thinking of, but also not quite right, is called a 'mega backdoor Roth', which involves contributing to a 401k via a non-deductible contribution (which is not part of 24.5k/yr limit), then immediately rolling it over into a Roth 401k. It has to be allowed by the plan, but some plans even offer to do the rollovers for you automatically.

The Mega Backdoor basically lets you get an extra ~40k/yr of Roth contributions, if you can afford it.