My point is more as an engineer, for the next app. I deploy things to heroku, simple enough, but when I feel constrained and want to learn a bit more to manage

My point is more as an engineer, for the next app. I deploy things to heroku, simple enough, but when I feel constrained and want to learn a bit more to manage a little more things, where do I look? To answer your question tho, a lot of things. Heroku makes it difficult to customize your stack, server, etc. Once your company and systems grow, you want more control.