What is the initial conditions file (*.IC)? What information does it hold?
This is an (optional) initial conditions file, providing initial values for flow rates, soil moisture, etc. It is generally autogenerated by Flow. A typical use case would be to 1) get a Flow model set up (e.g. catchments defines, stream network defined, etc), 2) specify a .IC file in the Flow input file, 3) Run Flow for a year. Flow detects it the file does not exist, and writes it at the end of a simulation, so running for a single year gives Flow a chance to “spin up” and stabilize the flow rates, which are then stored as used in subsequent runs as initial conditions. If the file is present, it is used to populate initial conditions.
How to define ‘pools’? Where does ‘flow_model’ plug in get information of these soil layers?

Soil Layers (more correctly, soil water storage pools) are defined based on the needs of a particular model application and should broadly reflect the soil properties in the study area and any additional storages that should be modeled. Generally, we include an above-ground storage for the interception of rain and snow in the veg canopy, plus soil compartments representing discrete below-ground storage pools. These pools are defined in the 'catchments' ‘layer_names”, “soil_layers”, “snow_layers” and ‘veg_layers’ attributes. If you are using the HBV plugin to Flow, you need to specific some specific pools, as the HBV plugin includes some assumptions about soil compartments, as follows:

0Snow (above ground)
1Melt (above ground)
2Unirrigated soils (below ground)
3Irrigated Soils (below ground)
4Upper Groundwater (below ground)
5Lower Groundwater (below ground)

Related model parameters are in HBV.csv, which in your version assumes uniform parameters across space. This can be relaxed by providing additional rows in the HBV.csv file to represent the heterogeneity of the parameters.

What ‘init_water_content'?
It provides initial soil water content, if the initial conditions file is not defined.
Where do we find information about routing methods (Reach Routing, Lateral Exchange, Vertical Exchange?