packages/@vue/cli-service/lib/commands/build/demo-lib-js.html

<!DOCTYPE html>
<meta charset="utf-8">
<title><%- htmlWebpackPlugin.options.libName %> demo</title>
<script src="./<%- htmlWebpackPlugin.options.assetsFileName %>.umd.js"></script>
<% if (htmlWebpackPlugin.options.cssExtract) { %>
<link rel="stylesheet" href="./<%- htmlWebpackPlugin.options.assetsFileName %>.css">
<% } %>

<script>
console.log(globalThis['<%- htmlWebpackPlugin.options.libName %>'])
</script>