Solved
Where to insert third-party tags/scripts
I would like to insert the GTM tag in AEM, could someone help me?
In my response, I am assuming that your AEM pages is using the latest WCM Core Components V2 Page. Also the screenshot provided below is Google's documation on how to install GTM onto your page. https://developers.google.com/tag-platform/tag-manager/web
Assuming that your AEM pages is using the latest WCM Core Components V2 Page, I will be placing the GTM code as high as in the head on the page as possible, so I will place the code in line:19 of my overlayed v2 Page Component, (head.html) https://github.com/adobe/aem-core-wcm-components/blob/master/content/src/content/jcr_root/apps/core/wcm/components/page/v2/page/head.html
2) I will be placing the next installation code from Google GTM immediately after the closing of the <body> tag, so I will place the code in line:45 of my overlayed v2 Page Component (page.html) https://github.com/adobe/aem-core-wcm-components/blob/master/content/src/content/jcr_root/apps/core/wcm/components/page/v2/page/page.html