What Is a Notion Embed?
An embed is a Notion block that loads another web page inside your page. You type /embed, paste a link, and Notion draws whatever lives at that link right there in the document. It is Notion's own feature, not something a widget tool adds, and it works whether or not you use one.
Native and generic embeds
Notion has special handling for a handful of big services, so a Figma file or a YouTube video gets treated a bit differently. Everything else falls back to a plain frame, which sounds like the lesser option and is exactly what a widget wants.
What can go in one
Rough rule: if you can open a URL in a browser tab, Notion can probably embed it. The exceptions are sites that specifically refuse to be framed, and those get their own page in when Notion will not embed a site.
The embed is the door, the widget is what you send through it. Confusing the two is why people go looking for a Notion plugin store that does not exist.
Next stop, what a Notion widget is.
The same block elsewhere
Embedding is not a Notion invention. Coda has it, WordPress has an embed block, Google Sites has one, and so do xTiles and Craft. That is why a widget built for a Notion page works unchanged in any of them, and why the skill transfers if you ever move tools.
What differs is how strict each host is about height. Notion lets you drag freely. Some others fix the frame, which is worth checking before you design something tall.
One thing the embed block does not do is resize itself to fit its contents. Notion sets the frame and the widget fills it, which is why a widget can look cropped if the block is shorter than the thing inside it. Dragging the bottom edge fixes it.