Simply paste the HTML code from the site you want to embed. Use <html>,<style>, and <script> to employ HTML, CSS stylesheets and JavaScript. Many websites like social media platforms, generate code snippets that you can add into your site. Google Sites makes it easy to plug and play those kinds of widgets.
This option to embed URLs from the web will often yield two options: insert an entire iframe of the target URL in your page or create a preview, which shows a link with a thumbnail and description from the site.
Sometimes you can't embed due to provider site permissions. In that case the URL will display as a text link. In other cases, the site your embedding may require authentication. So, if the preview is blank, sign in to the site and try reloading the preview. Viewers need to be signed in to see the embed.