Layouts contain shared html markup for the main parts of your site (head, header, navigation, footer).
Define a custom layout for your page template by creating one and calling it:
src/
static/
ejs/
layouts/
navigation
default.ejs
<!DOCTYPE html>
<html lang="en-US">
<head>
...
</head>
<body>
<%- include('../layouts/navigation') %>
<%- body %>
...
</body>
</html>