Fortnite is more ambient because it’s all dynamic lighting, which doesn’t support the intricate details available with baked lighting. If you’re using baked lighting, you have a lot more options to achieve different results, like increasing your ambient light sources direct(or indirect) intensity, increasing specific light ambient intensities, disabling shadows on the Sky Light, etc.
As for the auto-exposure, you can disable it in the Post-Process Volume or project settings. Or if you want to adjust the range that it adjusts down or up to, or increase the time it takes to adjust, you can do that in the Post-Process Volume.
Edit: I forgot, if you’re really want a flat ambient across the map, try disabling the Sky Light altogether and use the Environment Color in the World Settings. Or use a white cubemap texture in the Sky Light and tint it with the Sky Light settings.