When you have a button on a page, use the Content Selector on the right-hand side. You'll see a bunch of options, and the last one is modules. Click the button and then click the module to which you want it to navigate.
You can read more about the Content Selector here: What is the Content Selector in the Rich Content Editor? The guide doesn't specifically address Modules, but by expanding the Modules tab in the Content Selector, you'll be able to select a link to a specific module.
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