Ruby on Rails

In some apps I've worked on Rails 5.1 and prior, environment variables, saved directly into the source code.

In rails 5.2 Custom credentials encourages us to check-in only the encrypted version of our configuration, and keep our master.key keyfile outside of our repository. 

My question is this: Is there a way to segregate by environment? (i.e., development, staging, production?)

seems like the instructions for setting up AWS keys, for example, would have the dev, staging + production all pointing to & using the same AWS bucket, access key, and secret. But it seems like for many services I'd want to have different credentials for different environments. 

I found this SO post that discusses this question, but unfortunately it doesn't present a very good answer IMHO because the there are only two answers: 1) I don't quite understand and 2) a suggestion to basically check all your ENV variables against each of your environments, which seems like it could encourage a messy setup. I much like answer #1 from this SO post, but I don't understand how to implement it practically. 

https://stackoverflow.com/questions/53642152/how-to-manage-credentials-for-different-environments-in-rails-5-2

any tip appreciated. 

Thanks,

Jason