How to find comprehensive agreement before getting credit card?

For the Scotia card, at the bottom of the page you linked is another link labelled Terms and Conditions to this PDF: https://www.scotiabank.com/content/dam/scotiabank/canada/common/documents/pdf/Momentum-Infinite-Visa-Eng-Ekit.pdf#page=12

The bulk of the PDF is about the insurance that comes with the card, but pages 10 through 13 are about the cash back. Among other things, it says:

Your annual Scotia Momentum Visa Infinite cash back reward will either be deposited into an eligible Scotiabank Canadian Dollar personal chequing or savings bank account (“Eligible Account”) or it will be credited to your credit card statement.

So, yes, you get the cashback once a year. This PDF looks like the sort of "fine print" brochure that would come in the envelope with your actual card, and it's pretty comprehensive. I can't find anything in there about cancelling, but I would expect that you can cancel just before you are charged the next year's annual fee and that would be that. You can certainly call and ask them just that; they should not lie. I have cancelled cards shortly before they charge the annual fee and no-one has said I can't or I have to pay the fee.

For the Amex, towards the bottom there is a link that says Footnotes and that pops up a window that says, among other things:

Cash back is applied as a credit to your account annually.

So this too is once a year. This little window is not as useful as a separate searchable PDF, but it should help at least somewhat.