There are two ways to embed any site without plugin.
1) Very first and classic way to embed any site is using iframe tag.
In this classic method, all you need to do is define the height, width, and source to embed any URL.
2) Another way to embed website is using oEmbed tag
With OEmbed tag, you can even style the container with CSS. It also doesn’t require a closing tag and all major browsers support it.
Which is one of faster and easier way to embed content into WordPress.