creating AEP schema via API

To create a schema in AEP using an API, you'll have to use the schema registry API, You will have to make a POST API call to the required endpoint, with the schema definition in body.

The Schema Registry is used to access the Schema Library within Adobe Experience Platform. The Schema Library contains resources made available to you by Adobe, Experience Platform partners, and vendors whose applications you use. The registry provides a user interface and RESTful API from which all available library resources are accessible.

The Below Product Document will help you to create the Schema From Schema API.
https://experienceleague.adobe.com/en/docs/experience-platform/xdm/tutorials/create-schema-api#assign-a-class

https://experienceleague.adobe.com/en/docs/experience-platform/xdm/api/schemas#create

Further you can refer to the Adobe's Developer Portal: https://developer.adobe.com/experience-platform-apis/references/schema-registry/#operation/createSchema