The triangle in the Figma is one embodiment of the polygon. There is no hotkey for calling it; you need to call it from the drop-down toolbar in which the rectangle is selected by default.
A polygon can contain from 3 to 60 sides, in which 3 is a triangle, and 60 is actually an ellipse. 4 — square.
You can change the number of sides both in the settings panel and by pulling the Count mark on the figure. You can also change the rounding of corners in the settings panel and using the Radius label directly on the figure.
The sides of the landfill are not necessarily equal in length.