Defender for Cloud uses the Microsoft Monitoring Agent currently (vs the new Azure Monitor Agent). The recommended deployment method for hybrid (non-Azure) is using Azure Arc. Defender for Cloud will deploy the MMA agent to all servers in the subscription (and on Azure Arc) using an extension. This will also activate Defender for Endpoint using an extension. Hybrid Windows servers onboarded using a stand-alone MMA install and Linux servers may require separate MDE activation.
Defender for Endpoint uses the built-in Defender AV service as an agent. There is no agent to install on modern Windows OS. It is simply a matter of activation the service included in the OS. MDE also relies on policy deployed through GPO, Intune, or MECM to configure the antivirus agent settings and the OS hardening recommendations (ASR). Even though Defender for Cloud may activate MDE you will still want to configure these policies separately. MDE will also provide recommendations to improve OS hardening.
To verify your onboarding start by using the device inventory on the Microsoft 365 Defender portal. There is an onboarding section under Settings>Endpoints to assist you with any additional onboarding if needed.