You can find all the dependencies code in the vendor folder. That's where composer install them.
You can also run composer info to find every package installed, in case you need a link for each individual source/license.
Usually for me when you need to "give the code", it means "all the code you wrote yourself / the code you changed" (not sure what are your specific instructions). Including the source code for your own addons, a copy of config.php, a copy of composer.json and eventually a copy of the database should do it. You can build an exact replicate of your Flarum instance from that.
