Exam Guide: https://trailhead.salesforce.com/en/help?article=Salesforce-Certified-Heroku-Architecture-Designer-Exam-Guide

Videos:

Heroku Platform Basics for Partners – https://www.youtube.com/watch?v=PDTsT0e3evQ

Trailhead Resources:

https://trailhead.salesforce.com/en/users/equek/trailmixes/heroku-for-partners-beginner

https://trailhead.salesforce.com/content/learn/trails/heroku_enterprise

https://trailhead.salesforce.com/users/strailhead/trailmixes/prepare-for-your-heroku-architecture-credential