After you use Arc Rounded, add a new mesh line using the Mesh tool. If you don't see the Mesh tool, click the 3 dots at the bottom of the Tools panel and choose Advanced.
With the Mesh selected, select the Mesh tool and click in the center to add a new mesh line.
With the Anchor Point tool, click the bottom center anchor point.
With Pen tool, you can hold down Alt (Windows) or Option (Mac) and individually manipulate the handles that stick out on the left and right.
You can also repolition the anchor, but it is a delilcate situation so be careful as you are manipulating handles and anchor points.
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