The way it works to withdraw MFN is that it's a horizontal provision. It's done through an order in council and is effective for 180 days. It can only be renewed upon the agreement of Parliament.
For Canada, when you withdraw MFN, a 35% general tariff applies to virtually everything. There's a very narrow set of products that aren't included, like art objects. I don't know the exact ones, but they're not anything significant. Steel and aluminum were also included in this. This is in contrast to certain other countries that might have a more variable rate that applies when they take steps. Of course, theirs are structured differently within their law. It might require legislation, etc.
Part of the reason was simply that it was a horizontal measure done through an order in council and applied broadly.