This is a high level overview of the game before we go into detail.
The easiest way to think of the gameplay is as having 3 gameplay loops:
Encounter, Mission, and Campaign.
Most rogue-lites can be thought of in a similar way.
In Hades, for example:
These loops are arranged in a reverse-pyramid like this:

A campaign consists of missions.
Missions consist of encounters.
In the subsequent sections, I'll start with the Encounter Loop which focuses on the minute-to-minute gameplay of dealing with encounters (groups of enemies).
If you want more detail on how encounters work, you can skim this video brief on level design that covers the various spaces we use in Darkspy and types of encounters:
Once you're all set, you can go to the next section!