You can add custom code to the whole blog. This way, the custom code will be added to all templates in your blog. This is useful for adding services like analytics.
Console → Settings → Custom Code
You can also add custom code a specific post only in Post Settings. The custom code will then only be added to that post.
Console → Post → Advanced → Head/Foot Code
You can also define custom code for a tag. Then, this custom code will be added to the posts that has this tag.
Console → Settings → Tags → Edit Tag → Custom Code
In addition to these methods, you can also edit the theme directly.