How do you retrieve your data if it’s in the cloud

Make sure you are signed into iCloud and that the feature for the type of data you wish is active.

This document basically tells you how to sign into iCloud --> Set up iCloud on all your devices --> Sign in to iCloud on all your devices - Apple Support

Change your iCloud feature settings [iOS and Mac] - Change which apps sync and store data with iCloud - Apple Support -

There is no one method. It depends upon the type of data and which part of iCloud it is stored in. In general use the app that's appropriate for that kind of data. For photos and home videos, use the Photos app. It also depends on to what device you wish to retrieve this.