zebraidx and zebrasrv are both driven by a master configuration file, which may refer to other subsidiary configuration files. By default, they try to use zebra.cfg in the working directory as the master file; but this can be changed using the -c option to specify an alternative master configuration file.
The master configuration file tells Zebra:
Where to find subsidiary configuration files, including default.idx which specifies the default indexing rules.
What attribute sets to recognise in searches.
Policy details such as what record type to expect, what low-level indexing algorithm to use, how to identify potential duplicate records, etc.
Now let's see what goes in the zebra.cfg file for some example configurations.