In general, NRA's are taxed only on US-sourced income, using form 1040-NR. 401(k) withdrawals are considered US-sourced, because you were working in the US when the contributions were made. That means you should expect your withdrawals of Roth-401(k) funds to be non-taxed and withdrawals of pre-tax 401(k) withdrawals will be taxed by the US even if you are an NRA.
Whether any of these withdrawals are taxed in your home country will depend on their laws.
Yes, there is mandatory 30% withholding on your taxable 401k withdrawal if you are an NRA, this is to encourage you to file your tax return, and you will get a refund if the actual tax is less than 30%.
I'm less clear about IRAs, but they seem to be considered taxable as FDAP income (Fixed, determinable, annual, or periodical income). Same 30% rule applies.
Of course, any tax treaty may modify these general rules, so it is important to know the country you are asking about.
