How to get custom output from an executed pipeline?

Hello Heather and thank you for your inquiry. Custom outputs are not an inbuilt feature at this time. You can request/upvote for the feature in the Azure feedback forum. For now, I do have two work-arounds.

Utilizing the invoked pipeline's runID, we can query the REST API (using Web Activity) for the activity run logs, and from there, the activity outputs. However before making the query, it is necessary to authenticate.REST call to get the activities of a pipelineFor authentication I reccomend using the Web Activity to get an oauth2 token. The URL would be https://login.microsoftonline.com/tenantid/oauth2/token. Headers "Content-Type": "application/x-www-form-urlencoded" and body "grant_type=client_credentials&client_id=xxxx&client_secret=xxxx&resource=https://management.azure.com/". Since this request is to get credentials, the Authentication setting for this request is type 'None'. These credentials correspond to an App you create via Azure Active Directory>App Registrations. Do not forget to assign the app RBAC in Data FActory Access Control (IAM).

Another work-around, has the child pipeline write its output. It can write to a database table, or it can write to a blob (I passed the Data Factory variable to a Logic App which wrote to blob storage), or to something else of your choice. Since you are planning to use the child pipeline for many different parent pipelines, I would reccomend passing the child pipeline a parameter which it uses to identify the output to the parent. That could mean a blob name, or writing the parent runID to a SQL table. This way the parent pipeline knows where to look to get the output.