In a Roth IRA, the tax savings at withdrawal come from optimizing your Roth / Traditional / Taxable withdrawals over your remaining lifetime.
The simplest comparison would be Traditional vs. Roth - if you take that marginal dollar from traditional, you'll pay your marginal income tax rate on it today. If you take it from Roth, you pay nothing. This has tax impacts both now and into the future.
So you estimate lifetime cash needs and tax implications and try to minimize over your remaining life. Actionably, people figure out what they need this year, what they want to take from Traditional, what they want to take from Taxable, then anything beyond that they then take out of Roth. If you've got room, you might even convert from Traditional to Roth up to the top of the tax-bracket that you're comfortable paying this year.
Kind of a very individual, in-depth analysis. Also probably best to err on the side of lowering taxes now vs. in the future for a lot of reasons, but post-retirement, you're looking at a life-time tax optimization problem.