While you're creating the rectangle, or immediately after creating it, use the arrow keys - up & down - to change the curvature.

There's no way to make rounded-corner polygons other than drawing them by hand as Richard suggests.

There's more info here: https://help.sketchup.com/en/article/3000199

-m

BUT: is it only me (Mac M1, Layout 2021, and earlier verisions), that in the "Pages" panel, it doesn't allow to rearrange pages in "miniature/thumbnail" view, but only in "list" view?? This is quite awkward! Rearranging pages being able to see the thumbnails would be veeeery useful (and obvious)!