In simple terms they understand.

If they’ve baked cookies with you, you tell them the daddy gives some of the ingredients, the mommy gives some of the ingredients, and then the mommy is like the oven, and makes a baby over 9 months in her belly.

There were no further questions for years, and my kids knew that they came from mommy and daddy, and that they grew in mommy’s belly. The older they get, the more specifics they get, and if they’re old enough to ask a specific question, they’re old enough to get a specific answer.