<details> HTML element
Click below to see the code which creates this kinda dropdown, it is the <details> element.
A <details> code example (click here)
<details>
<summary>A <details> code example (click here)</summary>
... too much recursion ... ;)
</details>
The <details> element "creates a disclosure widget in which information is visible only when the widget is toggled into an "open" state" MDN says.