Yes there is a cron job!
Start here for background reading: https://wiki.civicrm.org/confluence/display/CRMDOC/Bounce+Handling
And check to see that your: Fetch Bounces scheduled job is ON (and that you have a system cron running). Of course if you're testing (sending Emails that you know will bounce) -> then you can also hit Execute Now for this specific scheduled job.
I found all I had to do was to just follow the Documentation: https://docs.civicrm.org/sparkpost/en/latest/install/
Note Bounces are processed through a callback (no need for an email account dedicated to bounce processing), but CiviCRM only processes bounces for CiviMail-originated emails. CiviDesk is planning to extend bounce processing to transactional emails in the future as well.