Google Tag Manager
Before you start using Google Tag Manager, make sure that you understand the difference between Google Tag Manager and Google Analytics.
Google Analytics is the tool that provides reports about activity on your store. Google Tag Manager is a tool that triggers your tracking codes based on defined rules. Google Tag Manager doesn't replace Google Analytics. Instead, it can be used to add Google Analytics tracking codes (tags) to a website, deploy Google Analytics code snippets, and define rules for when each tag must fire.
It's recommended that you use Shopify's built-in integration to Google Analytics because Shopify has the necessary logic built in to record Google Analytics Enhanced Ecommerce data. Adding Google Analytics to Google Tag Manager in addition to using Shopify's built-in integration can result in duplicate tracking.
To use Google Tag Manager with an upgraded checkout, follow the steps outlined in the custom web pixels tutorial for Google Tag Manager.