The URL of a Dynamics 365 Online instance tells you the location of the instance.

Please refer the second part of URL, after organization name:

• North America – *.crm.dynamics.com
• South America – *.crm2.dynamics.com
• Canada – *.crm3.dynamics.com
• Europe, Middle East and Africa (EMEA) – *.crm4.dynamics.com
• Asia Pacific Area (APAC)- *.crm5.dynamics.com
• Australia – *.crm6.dynamics.com
• Japan *.crm7.dynamics.com
• India *.crm8.dynamics.com
• North America 2 – *.crm9.dynamics.com
• United Kingdom (UK) – *.crm11.dynamics.com
• Microsoft Cloud Germany – *.crm.microsoftdynamics.de

You will find some additional information about developer endpoints within this MSDN article: Download the endpoints using the Microsoft Dynamics 365 Developer resources page