1. withraw of after tax 401K, the contribution is not taxed because it is already tax when it is conributed. The earning will be taxed as usual income.
2. the after tax 401K is taxed just like any of your after tax brokerage account, with following exceptions
(a) it is part of your retirement account that is protected from lawsuit and creditors.
(b) it is part of your retirement account that is not counted during college financial aid calcuation.
(c) you can rollover after tax 401K contributions to a ROTH IRA, and the earning from after-tax 401K to a traidtioal IRA see https://www.irs.gov/retirement-plans/rollovers-of-after-tax-contributions-in-retirement-plans