To access RingCentral sandbox platform API, use https://platform.devtest.ringcentral.com.
To access RingCentral production platform API, use https://platform.ringcentral.com.
The platform domain is the same for any RingCentral account. But when you call an API, there is the path to identify which account to access, specified by the account id. If the API call is at the extension level, you must also specify the extension id. E.g. to send a SMS message:
https://platform.devtest.ringcentral.com/restapi/v1.0/account/[accountId]/extension/extensionId/sms
If you provide an app to server any RingCentral customer, you must create a public app so anyone can login with their own login credentials. The platform will detect the account id, extension id if you specify them as a tilde "~".
https://platform.devtest.ringcentral.com/restapi/v1.0/account/~/extension/~/sms