To get a device under the control of Azure AD, you have two options:
Registering a device to Azure AD enables you to manage a device’s identity. When a device is registered, Azure AD device registration provides the device with an identity that is used to authenticate the device when a user signs-in to Azure AD. You can use the identity to enable or disable a device.
When combined with a mobile device management (MDM) solution such as Microsoft Intune, the device attributes in Azure AD are updated with additional information about the device. This allows you to create conditional access rules that enforce access from devices to meet your standards for security and compliance.
Joining a device is an extension to registering a device. This means, it provides you with all the benefits of registering a device and in addition to this, it also changes the local state of a device. Changing the local state enables your users to sign-in to a device using an organizational work or school account instead of a personal account.
This blog post shows how to join your Windows 10 device on your Azure AD
Joining your Windows 10 device
Open Start menu, click Settings icon to open Settings app
Select Access work or school, and then select Connect from the Access work or school screen.
On Set up Add a work or school account tab, select Join this device to Azure Active Directory.
Type in your email address for your work or school account, and then select Next.
In the password page, enter your password.
It will take few seconds to register your machine.
On the Make sure this is your organization tab, review the information to make sure it’s right, and then select Join.
Once you are done will get the following message and Click Done
Now, you will be able to see your Azure AD account through which you have registered your device.