In Figma, you can use the Ellipse Tool to create circles. To create a perfect circle, hold down the Shift key while you drag. You can also use the Rectangle Tool to create circles by holding down the Alt (Option on Mac) key while you drag.
If you want to cut a perfect circle out of an image, you can use the Ellipse Tool to create a mask. First, select the image. Then, click and hold on to the Ellipse Tool and drag out a circle. The circle will be filled with black.
Next, click on the fill color in the toolbar and change it to white. Finally, click on the mask icon in the toolbar and select Inverse Mask. Or bring the image to the front to inverse the mask placement.