syncing pages on iMac

In Catalina, visit System Preferences : Apple ID : iCloud Drive : Options… panel and enable Pages, Numbers, and Keynote. Now, folders for these applications will appear on iCloud Drive for Catalina, on your new iMac, and you should have access to the comparable documents saved to those folders from the respective applications on your phone and MacBook.

The other issue is your new iMac may have shipped with Pages, Numbers, and Keynote added by Apple. These need to be "activated" before usage via the Mac App Store, after you have signed in with your Apple ID, and pressed ⌘O (oh) to View your account. This will present the old "purchased" panel, where you should be able to select each of these applications to "activate" them.

Apple also has an article, Share folders with iCloud Drive, which may prove useful. Don't overlook the available, online Pages User Guide for Mac, or the respective Pages, Numbers, and Keynote support sites.

Explain what you mean by syncing your Pages application to your iMac? Is this a Pages collaboration attempt, or file sharing?

What versions of Pages (Pages: About Pages) are using on the two Macs, and what operating system versions (menu : About This Mac) are running on both?

You still have not shared the Mac product description and operating system version from menu : About This Mac panel for both of your Macs. Which iPhone is it, and what version of iOS is it running from the Settings : Apple ID : iPhone panel?

This information is important as Pages documents track unsupported hardware, both Mac and i-Devices, and minimum operating system version numbers for opening/sharing documents.

There won't be any hardware, macOS, or iOS product limitations blocking sharing, but just what is causing the sharing issue, I have no idea at this point.