Here is a quick overview of the most important code parts. They can be found in the telemetry folder.
TelemetryController: Main telemetry logic, e.g. assembling pings, local storage (when archiving is enabled), preference changes, testing
Telemetry.cpp contains most of the public interface, implements the IDL
The different data types for telemetry are handled in TelemetryHistogram, TelemetryScalar, TelemetryEvent.
TelemetryEnvironment: A helper for gathering environment data, like build version or graphics data
TelemetryScheduler: Starts regular jobs for collecting and sending data
TelemetrySend: Sending and caching of pings
TelemetryStorage: Handles writing pings to disk for TelemetrySend
TelemetrySession: Collects data for a browsing session, includes many of the most important probes (aka metrics)
Policy: A layer of indirection added to provide testability. A common pattern in many files
pings/: Contains definitions and handling for most ping types, like EventPing
More details on different topics can be found in these chapters: