Do you have a full guide to help understand how Unity Ads works?

Yes! If you want to read about Unity Ads in detail, you can find our documentation site here:
https://unityads.unity3d.com/help/advertising/getting-started 

If you have any specific questions not answered by our documentation, you can contact support by emailing unityads-support@unity3d.com or using the Unity dashboard's help feature.