digitransit is a slightly complex system composed of multiple services in container that talk to each other. A nice architecture image can be found at the digitransit.fi documentation.
The main components of an digitransit deployment consists of:
Multimodal routing engine (OpenTripPlanner)
Address search (originally pelias)
Background map service (hsl-map-server: [tessera], [tilelive])
Web browser-based user interface (digitransit-ui)
A nicer description of the components and their job exists also at https://digitransit.fi/en/services/
As can be seen in the diagram, some dependencies are circular. This poses a few challenges when first setting up a working instance, which we will address later on.