There’s a really handy theme component to track down the plugin outlets of our various handlebars templates: Plugin outlet locations theme component. At the end of the first post is a link to a preview of the component on our theme creator site. You can use that to find most plugin outlets without even having to install it on your own site.
One thing to be aware of is that templates and widgets are two separate concepts. With that said, they can work together if needed by mounting a widget to a plugin outlet of a template. It’s more common to use connectors/components/controllers to work with plugin outlets at this point like you would in a plugin, though.
This section of the developers guide covers a lot what you are looking for. I would also recommend going through the GitHub repositories of theme components in our theme category to see some good examples.