To pass static values from a website into Google Analytics Custom Dimensions using the data layer, which of these must be set up?

To pass static values, you need to first make the data available. The data layer (window.dataLayer) is the recommended method for passing data from your website to Tag Manager. Once the data is in the data layer, you need a Data Layer variable in Tag Manager to read that specific value. Finally, to send this value to Google Analytics, you must configure a Google Analytics tag (Universal Analytics or GA4 Event/Configuration) and define a Custom Dimension within that tag, mapping it to the Data Layer variable you created. A Custom Segment is a reporting feature within Google Analytics used for analyzing data, not for collecting or sending it.