How do I create things, like pages, in more than one class at a time?
There is not away to create pages, modules in more than one class at a time. The best thing to do is to create either a "Master Course" for a single course or a "Master Course Template" (how you want all courses laid out) and get everything setup in them and then import the content from that into the other courses.
Besides the course import process that Brian mentioned, you might be able to use Blueprint Courses. There is additional information about Blueprint Courses in the Courses and Sections portion of the Canvas Instructor Guide. Another option is to publish the pages to Canvas Commons and then import it into those courses (Canvas Commons has its own section in the Canvas Instructor Guide).
There isn't a one-size-fits-all approach. You have to look at what you're wanting to do.
Consider whether this content will ever change during the course of a term when you look at the approach you use. The course import method doesn't allow you to update the page if it changes without going through the import again. The Commons approach can notify the instructor that the page has changed and allow them to update it, and the Blueprint approach allows you to push the changes out to the courses that have it.
It may sound like Blueprint Courses are the best approach, but they have restrictions as well. You can only have one blueprint master associated with a course. You cannot have some pages come from a Biology master blueprint course and other pages come from an institutional master blueprint course. In my opinion (not having used them), they are designed for when you have courses that are substantially the same.
Some institutions are looking to create institutional pages that are not tied to a specific course. For example, they may want to include Library Resources or Academic Honesty Policy in every course. If you're looking at a mass distribution, then you would want to look at automating things through the Canvas REST API. There is a Pages API that allows you to get a list of pages, create new pages, and update existing pages. This may be the quickest way to mass deploy to multiple courses and make sure that it's kept up to date, but it requires some programming skills and cannot be done through just the web interface.
Retrieving data ...