- Are you referring to Azure AD Connect? Then yes, Hybrid Azure AD needs to be enabled. Autopilot and the Intune Connector for AD will enable the device to join the on-prem domain, the Hybrid Azure AD Join process happens later in Autopilot or once in Windows
- That is most likely because you haven't enabled Hybrid Azure AD in Azure AD Connect options.
- No, that GPO isn't required. That is for existing devices.
I would strongly recommend reading through this link which will explain into more details. Especially how you end up with two devices, one Azure AD device and one Hybrid Azure AD device which is how its designed. https://oofhours.com/2019/07/15/inside-windows-autopilot-user-driven-hybrid-azure-ad-join/