You can find a number of in-built integrations in the website's integrations settings:
Some third-party tools and widgets can be added via Google Tag Manager or by simply using a custom code. There are two ways to add a custom code to your website:
To the <head>/<head> part of your website's source code
To the <body></body> part of your website's source code
Whether the code should be added to the <head> or the <body>, depends on the integration itself. Integration developers usually specify that in the instructions, for example, Google AdSense: