GTM Features Breakdown and Implementation Guide for Administrators?

Summary

What resources are available to help learn the features of GTM, and how those features work?

Content

I am looking to expand my knowledge about GTM which we have implemented in our company. (We do not use OTM, just GTM). However, everything I find either is so high-level that it doesn't provide anything valuable or it's related to an update and is written from the perspective that I already have knowledge that I don't have.

So I'm looking for  books, implementation guides, tutorials, blogs, etc. that might explain each of the features, how they're implemented (if not default) and ways to modify the default to surface additional information or modify the user workflow.