In your Google Sites editing mode, there are three sections.
In the top sections, you'll find Sites name, Undo/Redo, mobile and desktop view modes, sharing, settings, and version history, as well as options to copy and publish.
Secondly, the body section represents your website, and you can add contents to it.
Last but not least, all components, pages, and themes are represented in the right sidebar.
* Also, read: How to Design the Perfect Header and Footer for Your Google Sites - Tips and Tricks
Adding contents and components are pretty straight forward.
Under the ‘Insert’ menu pane, you can find four types of contents: Text, Images, Embed or Drive.
Text: Add text based contents under the text box.
Images: Add images to the site.
Drive: Choose files from your Google Drive and embed within the site.
When you double-click on Google Sites body, you will notice a circle with options such as Embed, Images, Text, Upload, and From Drive.
Content Blocks: You can click to add layouts like two columns and four columns.
Additionally, you can add many components within Google Sites, such as buttons, image carousels, social links, YouTube videos, and Google Calendars.
Google Sites is already optimized for mobile devices
The good thing about Google Sites is that you don't have to build a different design or layout for mobile. Contents and components that you created on desktop are automatically optimized and resized for mobile devices. No need to worry about messy looks on mobile screens.
How to publish your website
Once you are done creating your website, you need to publish it. Click the [Publish] button on your upper right corner and enter the public name of the site. It will become your permanent google site name under sites.google.com default link. Once you decide your public site name and then you can connect this site url to your actual domain.
Your Ultimate Guide to Setting Up a Custom Domain on Google Sites
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The above guide is just the tip of the iceberg and you can find much detailed information on Google's official "Google Sites training and help" page so I will avoid redundancy.