Google analytics 4 - What's the difference between Google tag and Google Tag Manager (GTM)
In short:
Consider Tag Manager if you need:
- The ability to deploy and modify both Google and 3rd party tags
- The ability to deploy and modify tags for both web and mobile apps
- The ability to deploy and modify tags on the fly from a web interface
- Collaboration and versioning capabilities
Consider gtag.js if you are unable to use Google Tag Manager, or the following capabilities are important to you:
- The ability to install tags directly on your web page, without the ramp-up time of setting up a tag management system
- The ability to work with your tags directly in JavaScript without being required to work with a separate interface
Source: https://support.google.com/tagmanager/answer/7582054