How can I leverage Azure AD B2C to create and manage users through a REST API?

If you want to manage Azure AD B2C users with Rest API, you can use Microsoft Graph API to implement it. For more details, please refer to the document and the document

For example

• Register application

  1. Sign in to the Azure portal.
  2. Select the Directory + Subscription icon in the portal toolbar, and then select the directory that contains your Azure AD B2C tenant.
  3. In the Azure portal, search for and select Azure AD B2C.
  4. Select App registrations (Preview), and then select New registration.
  5. Enter a Name for the application. For example, managementapp1.
  6. Select Accounts in this organizational directory only.
  7. Under Permissions, clear the Grant admin consent to openid and offline_access permissions check box.
  8. Select Register.
  9. Record the Application (client) ID that appears on the application overview page. You use this value in a later step.

• Grant API permissions

  1. Under Manage, select API permissions.
  2. Under Configured permissions, select Add a permission.
  3. Select the Microsoft APIs tab, then select Microsoft Graph.
  4. Select Application permissions.
  5. Expand the appropriate permission group and select the check box of the permission to grant to your management application. For example: Directory > Directory.ReadWrite.All: For user migration or user management scenarios.
  6. Select Add permissions. As directed, wait a few minutes before proceeding to the next step.
  7. Select Grant admin consent for (your tenant name).
  8. Select your currently signed-in administrator account, or sign in with an account in your Azure AD B2C tenant that's been assigned at least the Cloud application administrator role.
  9. Select Accept.
  10. Select Refresh, and then verify that "Granted for ..." appears under Status. It might take a few minutes for the permissions to propagate.

• Create Client secret

  1. Under Manage, select Certificates & secrets.
  2. Select New client secret.

• Get access token

POST https://login.microsoftonline.com/<your b2c tannat name >/oauth2/v2.0/token 

Content-Type: application/x-www-form-urlencoded

grant_type=client_credentials
&client_id=<your sp appId>
&client_secret=<you sp password>
&scope=https://graph.microsoft.com/.default

• Create User

POST https://graph.microsoft.com/v1.0/users
Content-type: application/json
Authorization: Bearer <access_token>

{

      "displayName": "[TEST] Bridgette Harmon (Local account)",
      "givenName": "Bridgette",
      "surname": "Harmon",
      "identities": [
        {
             "signInType": "userName",
             "issuer": "<your b2c tenant>",
            "issuerAssignedId": "Bridgette"
        }
      ],

      "passwordProfile" : {
          "password": "password-value",
          "forceChangePasswordNextSignIn": false
        }
    }

Like you, I am looking for a centralized way to keep my users and to perform Single Sign On from multiple applications, or api. I am leaning towards developing my own Identity Provider at this point vs Azure AD for a cost basis standpoint. Good Luck.

To amplify @JimXu answer, use this client as an example.

"This approach is similar to a service account scenario where the application acts as itself, not as a user that signed-in via an interactive user login. This is done by using the OAuth 2.0 client credentials grant."