OTP releases are not a lot harder to understand than most OTP applications you’ll encounter in the wild. A release is a set of OTP applications packaged in a production-ready manner so it boots and shuts down without needing to manually call application:start/2 for any app.
Of course there’s a bit more to releases than that, but generally, the same discovery process used for individual OTP applications will be applicable here. You’ll usually have a file similar to the configuration files used by systools or reltool, which will state all applications part of the release and a few 8 options regarding their packaging.
To understand them, I recommend reading existing documentation on them. If you’re lucky, the project may be using relx 9, an easier tool that was officially released in early 2014.