ES is an app that sits on top of Splunk that provides insight into security data, which can come from anywhere depending on your use-case. The app includes various methods to collect and visualize data, but any 'monitoring' is dependent on the searches that you configure, and they will depend on what you are looking for.
The parsing of data is handled by splunkd, according to rules that can be put in place by any app. If you are looking at a 'standard' log, then it's likely we already have parsing rules included by default, but for custom logs you will likely have to define some if Splunk's initial attempts don't work for you
To set up a conversation specific to security use-cases, I recommend engaging your Sales Rep to set up a call with someone who can assess the use-case(s) you have in mind and provide