"丸い" is 3D(three‐dimensional shape). "円い" is plane.　 A.丸い石を拾いました( I piked up the spherical stone ) B.円いテーブルを買いました。( I bought round table. ) However, we rarely use "円い". We mainly use "丸い" both Case A and Case B". "女の子" mean same as girl in English. "女子" mean same as women in English. And, "女性" mean same as female in English. I hope this makes sense.