When you want to temporarily close your store to the public, you can replace the default page and content with the one of your own choosing. Editing the maintenance page is a simple matter of going to Information > Pages in the admin area, and editing the page called "Maintenance". On the maintenance edit page, you can add your content in the Description text area.
If you'd like to go further and add blocks to it, such as a slideshow, products, social media feeds, and much more, and reorganized sections, then, in the maintenance edit page, click on the Design tab and assign a layout to it. If there's already a layout titled "Maintenance", you can skip this step. If there isn't one in the dropdown field, then, save this page, and follow the documentation linked above to create a new Layout called Maintenance. You can then assign that layout in the Maintenance edit page's Design tab.
Next, go to Design > Theme Settings > Layouts Builder and select Maintenance in the dropdown field to load that layout. Now, you can add nearly any block available to that page and it will be displayed on the maintenance page in Maintenance mode.
To enable/disable maintenance mode and to learn how to use the Store Access Key to allow yourself and others to access the frontend without logging in to admin, visit this knowledge base article: How do I set maintenance mode and how can I access my site while in maintenance mode?