httpd.conf is Apache's main configuration file by default. It will then call out other files and directories via Include and IncludeOptional directives which is meant to simplify the main configuration file, and the structure wildly varies between platforms.

httpd.conf itself is located differently between platforms. These are some of the known locations;

If your platform of choice is not on the list, simply run httpd -V from the terminal and look for SERVER_CONFIG_FILE.

# httpd -V
Server version: Apache/2.4.25 (Unix)
Server built:   Feb  6 2017 20:02:10
Server's Module Magic Number: 20120211:67
Server loaded:  APR 1.5.2, APR-UTIL 1.5.4
Compiled using: APR 1.5.2, APR-UTIL 1.5.4
Architecture:   64-bit
Server MPM:     prefork
  threaded:     no
    forked:     yes (variable process count)
Server compiled with....
 -D APR_HAS_SENDFILE
 -D APR_HAS_MMAP
 -D APR_HAVE_IPV6 (IPv4-mapped addresses enabled)
 -D APR_USE_FLOCK_SERIALIZE
 -D APR_USE_PTHREAD_SERIALIZE
 -D SINGLE_LISTEN_UNSERIALIZED_ACCEPT
 -D APR_HAS_OTHER_CHILD
 -D AP_HAVE_RELIABLE_PIPED_LOGS
 -D DYNAMIC_MODULE_LIMIT=256
 -D HTTPD_ROOT="/usr"
 -D SUEXEC_BIN="/usr/bin/suexec"
 -D DEFAULT_PIDLOG="/private/var/run/httpd.pid"
 -D DEFAULT_SCOREBOARD="logs/apache_runtime_status"
 -D DEFAULT_ERRORLOG="logs/error_log"
 -D AP_TYPES_CONFIG_FILE="/private/etc/apache2/mime.types"
 -D SERVER_CONFIG_FILE="/private/etc/apache2/httpd.conf"

# httpd -V | grep SERVER_CONFIG_FILE
 -D SERVER_CONFIG_FILE="/private/etc/apache2/httpd.conf"