External system can access the Appian interface via

Embedded Interface
Portals

Embedded Interface :

Embedded interfaces allow Appian interfaces to be embedded in external web pages. This lets users interact with Appian forms and interfaces directly within those external pages, without navigating to a distinct Appian environment like a site. In fact, users may not even realize they are interacting with an Appian-powered interface, especially if a custom theme is applied.

The following types of Appian interfaces can be embedded in external web pages:

Tasks
Actions
Related actions
Reports (including task reports)
Record views

Learn more about it here

Appian Portals:

It connects external users to the information and processes that you already have in Appian. With Appian Portals, you can use low-code to build public websites that are conveniently connected to your Appian applications.

Public users can use portals to submit forms, file claims, view documents and information, and more. All without requiring them to log in.

Configuring interface in portal

Create a new portal object > Click on add page -> you can add the interface which you want to expose to the external system