How to create a custom API

Hi.
Can you tell me if it’s possible to create an entire custom API for specific needs of a company?
Context: a company that use the CRM Hubspot wants to allow an external partner to send JSON or XML datas (whatever the format) to a specific API Hubspot url that be able to read these specific datas and send it to their inbox Hubspot.
So we need to develop specific Hubspot API that be able to read datas that this external partner send.
Is there a way? Do we have to create an app for that or there is another way to do this specific needs?
Sorry fot these questions. We’re new to Hubspot and we didn’t find the answer in Hubspot developer documentation.
Thanks in advance for your help.

For Data transfer you can go with app availabe in app marketplace at HubSpot and also Zapier tool will help on that.

Thanks for your answer.
We’ll take a look to these 2 solutions.

Hi.
The 2 solutions proposed by @Endhrakumar do not seem to correspond to our need unfortunately.
Indeed, we need to be able to code an API function ourselves somewhere in Hubspot that will process a Json received from an external party and send an email with this information received in the “Inbox” part of the Hubspot CRM of the client.
Would this process be doable in any way with Hubspot?
Thanks in advance