How can I use either my iPhone or iPad to collaborate using Pages or Numbers?

Hello

You can do that, but you will need to be using a few things.

√ The Numbers and Pages app on the iPhone and iPad

√ Us the same iCloud account, on all devices

√ Have iCloud drive turned on, on all devices

Open the app from the device.

Select the "+" and choose the iCloud.

Note an iCloud account is a single user account not a multiple user database.

Have fun

I think true simultaneous collaboration is only possible via web browser, which means someone has to be using a Mac or PC, since iOS devices cannot access icloud via a browser.