The first part of understanding the three types of sources is understanding the structure and relationship between the user, session, and event.
In the following drawing, we can see the structure of user–>session[s]–>event[s]

ga4 event structure

The source type filtering that can be found in GA4 is divided into 3 main types:
1. First user source: self-explanatory.
2. Session source: self-explanatory.
3. event source, AKA Source / Medium: most vague one.

Notice that this one belongs to the Attribution section in GA, and thus, when on the event level, the source is calculated by the attribution model. if, for instance, the attribution model selected on the GA4 instance is data-driven, the source will be calculated by the platform, suggesting that this was the source selected to get the attribution. This will explain why the number of the aggregated number of events by source is not necessarily a whole number and is likely to be of a double value.
[445.45, for example, rather than 445.0]

This is relevant for all the attribution-related dimensions, which also contain medium, campaign, and term