You can have a look at the fscomm/ directory in FreeSWITCH git repo, then peak at fshost.cpp (https://freeswitch.org/stash/projects/FS/repos/freeswitch/browse/fscomm/fshost.cpp)

Pay particular attention to the FSHost::run() method calling the following functions:

switch_core_init()

switch_core_init_and_modload()

switch_core_runtime_loop()

That's the gist of it to get started. In my opinion most of the time is better to launch a lean freeswitch process (with only the modules you need in your app) in the background and control it using ESL

Cheers

Moy