hi! The geometry API provides the methods to do analysis (the backend calculation stuff). The actual drawing and rendering will be done with Unity's API since that's what game engines are specialized in.
If you just want to draw rectangles and triangles you may not need the geometry API, as long as you know the lat long or the world transform of the vertices, you can draw them with Unity's API. For example, drawing triangle: https://docs.unity3d.com/ScriptReference/GL.TRIANGLES.html
If you do need to utilize the analysis function of the geometry API, please checkout our measurement sample https://developers.arcgis.com/unity/sample-code/Measure/ to see how you can show results of the geometry API (in this case, calculate distance between points and render a line between them).
To see the geometry API and its capability, you can check the geometry API reference https://developers.arcgis.com/unity/api-reference/ (left side, under the geometry category.)
