Generally, yes, they are subject to US tax.
The 401k is subject to US tax because pensions are taxed according to where the work was performed that earned the tax.
Annuities are subject to tax because they fall under the category of "Fixed, Determinable, Annual, or Periodical (FDAP) income". (Now that I think about it, even if a 401k is not technically a pension, it would then fall under the category of an annuity.)
30% withholding by the plan trustee is mandatory.
If you file a return and pay the tax (which will be equal to or less than the withholding, so you might get a refund), you might also be able to claim a credit in your home country for the foreign tax paid on the same money.
https://www.irs.gov/retirement-plans/plan-distributions-to-foreign-persons-require-withholding
https://www.irs.gov/individuals/international-taxpayers/fixed-determinable-annual-periodical-fdap-in...
https://www.irs.gov/individuals/international-taxpayers/effectively-connected-income-eci
https://www.irs.gov/individuals/international-taxpayers/taxation-of-nonresident-aliens
You can't use the standard deduction, you would file according to the rules for non-resident aliens. This is a form 1040-NR and Turbotax does not support this form, you will need to find another provider.
Specific to Canada, there is a tax treaty between the US and Canada that may modify the usual rules.
https://www.irs.gov/pub/irs-pdf/p597.pdf
You may want to speak to an expert.
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