Queueable Apex callouts

Salesforce apex Queueable is allowed you to invoke the external systems by using callout and the below sample code shows how to make callouts from the Queueable apex.

Sample Code : 

This code is populating the  County currency based on the mailing country name  on the contact from external rest API by using callouts

Remote Site Setting 

Please add the remote site setting. If you are using named credentials, you can skip this step

Invoke the Queueable Job 

You can invoke the Queueable job with below code the execute anonymous salesforce

Monitoring Job

after you submit your queueable class for execution, the job is added to the queue and will be processed when system resources become available. You can monitor the status of your job programmatically by querying AsyncApexJob or through the user interface in Setup by entering Apex Jobs in the Quick Find box, then be selecting Apex Jobs or By Running SOQL query on AsyncApexJob object as shown below.