Step-01: GTM base code to header (theme.liquid file)
Online Store----> Themes ----> Actions ----> Edit Code ---->
Step-02: Create the dataLayer-allPages snippet ( use exact naming and casing) & copy and paste the below code
Go to Online Store ----> Themes ----> Snippets ----> Add a new snippet
Name: dataLayer-allPages
Shopify All Page Data Layer Snippet Code
Step-03: Just call dataLayer-allPages this page on header (theme.liquid file)
Login into Shopify admin panel:
Online Store----> Themes ----> Actions ----> Edit Code ----> Layout ----> theme.liquid (File) ---->
> Put the Data Layer call code directly below the opening <head> tag
{% include 'dataLayer-allPages' %}
Step-04: Put the bellow code on Checkout or Purchase Confirmation Page
Go to Settings ----> Checkout ----> Order processing ----> scripts (Additional)
Code for Checkout page: Checkout Code