What is Sauce Labs
Sauce Labs also has a very good documentation of setting things up with different languages, testing frameworks and CI providers.
How the setup works
Sauce Connect creates a secure tunnel from your environment to Sauce Lab’s environment. After the connection has been established, virtual machine with a certain browser in Sauce Labs connects to your local environment’s server which serves the test page.
This what it looks like in practice:
Local environment or customer environment can also mean CI environment. Most of the time you just want the CI to run tests with all configured browsers in Sauce Labs, but you also might want to run tests with a certain browser from your local machine.
JS Unit testing
Sauce Labs supports Karma test runner out-of-the-box. You should first get your tests running with Karma before trying to run them in the cloud. Karma supports a lot of testing frameworks such as Mocha and Jasmine. You can also use their raw REST API to use any test runner, but it’ll probably be easier to just settle with Karma.
I don’t see that much benefit of the manual testing but it’s good for just testing if your page works at all in a certain browser