When you have a sandcastle open, then there’s that “Open in New Window” button that will open that sandcastle in a new window. In this case, the sandcastle source code will be part of the URL, like
https://sandcastle.cesium.com/standalone.html#c=tVZtb9s2E....J7T/wA
with that last part (c=...) being a representation of the sandcastle code.
You can still create a standalone HTML file that contains “the Sandcastle code” in a script tag:
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<script src="https://cesium.com/downloads/cesiumjs/releases/1.120/Build/Cesium/Cesium.js"></script>
<link href="https://cesium.com/downloads/cesiumjs/releases/1.120/Build/Cesium/Widgets/widgets.css" rel="stylesheet">
<title>Example</title>
</head>
<body style="background-color:#AAAAAA; color:white">
<div id="cesiumContainer" style="width: 1000px;"></center>
<script type="module">
// Your "Sandcastle" code going here...
</script>
</body>
</html>
(This is what your other thread is about…)