Text notes can "include" another note as a read-only widget or an interactive widget, depending on the type of note.
This can be useful for e.g. including a dynamically generated chart (from scripts & "render HTML" note) or other more advanced use cases.
In the Formatting toolbar, look for the button. There is also a keyboard shortcut defined for it but it is not allocated by default.
An attachment can be included the same way, from the context menu of a link to it (see Embedding an attachment there).
If a shared note contains one or more included notes, they will be displayed in the content of the note as if they were part of the note itself.
For this to work, the included notes must also be shared, otherwise they
will not be shown. However, the included notes can still be hidden from
the note tree via #shareHiddenFromTree.
Since v0.104.0, included notes might become interactive depending on their note type: