You are correct in understanding that some of this is normal, and even the finger experimentation is normal. Little ones often stick their fingers anywhere there is a hole that the finger fits into whether on their own bodies, someone else's body or even a pet.
Because this is a behavior that you do not want to encourage or see more of, follow your instincts and have her stop by telling her not to do that. Take her hand away from the area and redirect her attention elsewhere. Give her the soap or the washcloth and have her wash her toes -- something that keeps her hands busy on a different area of her body.
It’s also time to talk with her about her "private parts." Since she’s becoming more aware of those parts, it’s necessary to keep herself safe by labeling them "private" and teaching her what that means. Explain that others should not see or touch them. Have this conversation with her at bath time to make sure she remembers what parts are private and use the opportunity to ask if anyone has touched them.
