Check out the doc site here apex-enterprise-patterns.dev. It houses seventeen chapters on Separation of Concerns in Salesforce and implementing it with the Apex Common Library. Every chapter has a companion video, and the whole thing is searchable.
Housed in this repo is also an older version of the wiki. It is no longer maintained. The site above is the maintained version.
There is also some example code showing you how an application (in this case a help desk app) in a Salesforce org might separate its concerns. If you reference the code while reading the guide it should aide considerably in learning how to leverage the Apex Common Library to implement Separation of Concerns in your Salesforce org.
If you believe there is any information missing from this guide or that it needs more info in certain places, please submit an issue on this repo here and I'll add it ASAP!
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