How to use this confing function
Without looking at the library, you can check yourself if this method (INA3221SetConfig) is called from the begin method (of the same class). Just trace back the methods manually until you find a call to this method.
About the | or 'bit-wise-or' operator (see https://en.wikipedia.org/wiki/Bitwise_operations_in_C), these are used to set bits in a byte/word/long.
Most registers have 8, 16 or 32 bits and with the | operator you can easily set multiple bits at the same time. Each value (e.g. INA3221_CONFIG_ENABLE_CHAN) has a value that is a power of 2 (mostly defined in the form 1 &lt;&lt; ..something.. you can add them (with +) or | them together, but | is more clear what the purpose is and repeatedly calling | the same bit twice is not a problem (it still gives the same result).