Can anyone tell me exactly how changes work in the editor interface? So I've plugged Optimize into the code of my site, but how is it rendering changes on my site without access to the CMS? Is this being done through laying CSS over the top of my page, or what? Having a hard time finding the answer to this. Thanks!
The Optimize editor is just preparing a list of changes that you want to do (i.e. change the color of a button, or change the text of a header) which are then saved in your experiment, under your Optimize container.
In order to run your experiment (for your users), you need to include the Optimize snippet in your page (actually, before starting your experiment for your real users, you may want to test it first yourself, using the preview functionality).
This snippet will then download the Optimize container script, which implements the modifications on the user's browser (you are right, on top of your actual page that is served by your CMS).
The Optimize runtime does all necessary hiding (if your follow the recommended instructions) so that your users will not be able to see the original page before it gets changed.