No code approach:

Using Automation Rules you can create webhooks that allow to define processing logic when an external party calls that webhook's URL. See https://www.odoo.com/documentation/18.0/applications/studio/automated_actions.html

XML and JSON RPC:

You can take a look at https://www.odoo.com/documentation/18.0/de/developer/reference/external_api.html which holds the documentation for the external API (XML/JSON RPC). Using this, you don't actually need to specify an endpoint since there is just a common one that basically allows to use every model and function already.

Custom controller:

Dedicated API Endpoints can be achieved by creating dedicated controllers (https://www.odoo.com/documentation/18.0/developer/reference/backend/http.html) in a custom module.

REST API:

A more sophisticated integration can be done by utilizing OCA's fastapi integration (https://github.com/OCA/rest-framework).