...
index.php (first time setup coming from app/assets/scaffold/files/index.php)
.htaccess (first time setup coming from app/assets/scaffold/files/.htaccess)
app → Core Mautic application coming from mautic/core
themes/contrib → themes downloaded by composer (public)
themes/contrib → themes downloaded by composer or custom written (private)
plugins/contrib → plugins downloaded by composer
plugins/custom → plugins from own packagists or custom code
media/ → all media files similar to today including the different folders
config/ → Configuration (user created)
vendor/ → All dependencies from composer
Alternative folder structures possible with composer
web/index.php (first time setup coming from app/assets/scaffold/files/index.php)
web/.htaccess (first time setup coming from app/assets/scaffold/files/.htaccess)
web/app → Core Mautic application coming from mautic/core
web/themes/contrib → themes downloaded by composer (public)
web/themes/contrib → themes downloaded by composer or custom written (private)
web/plugins/contrib → plugins downloaded by composer
web/plugins/custom → plugins from own packagists or custom code
web/media/ → all media files similar to today including the different folders
config/ → Configuration files
vendor/ → All dependencies from composer
...