Google Analytics is a repository and schema that holds all the data from user behaviors on your site.

Google Tag Manager is a centralized tagging management system that allows you to build tags that pull data from your website and send the data to a 3rd party vendor, i.e. Google Analytics.

Looker Studio (used to be Google Data Studio) is a visualization program that allows you build charts, graphs, tables to from datasources such as Google Analytics and BigQuery.

BigQuery is a cloud warehouse that allows you to save a copy of all your GA4 data - unfiltered and unsampled. Then you can use SQL to pull in-depth insights about your site users.

The dataLayer is a virtual location where a developer can pass events and variables from the site. Google Tag Manager then can grab these events and variables and send them to GA4/BigQuery for deeper insights.

A/B Testing is a great way to test an idea without involving development. Change the text of an important CTA, modify a link's destination url, change the color of a background - it can all be done through A/B Testing.