Do I have to pay Canadian tax every year on growth inside my US 401(k) before I withdraw it?

No, not under the normal treaty-based treatment. The growth inside a US 401(k), investment income, capital appreciation, and so on accumulating within the account, is generally not taxed by Canada year by year simply because you're now a Canadian resident, mirroring how Canada treats the growth inside an RRSP as untaxed until money actually comes out. Without this treaty protection, a Canadian resident with a US retirement account could otherwise face the odd result of being taxed annually on paper growth in an account they can't yet access.

This favourable treatment generally depends on the arrangement genuinely qualifying as the kind of plan the treaty covers, and has historically required an election or specific disclosure to be made with your Canadian tax filing rather than being something CRA simply assumes applies without any action on your part. The requirements can also differ somewhat between different types of accounts, so confirming exactly what your specific plan requires, rather than assuming all US retirement accounts are treated identically, matters.

Canadian tax generally does catch up with this income eventually, when you actually withdraw funds, which is a separate question from whether the annual growth itself is taxed along the way.