Webform custom Options file location
Webform options are config entities. You can export and import then via /admin/config/development/configuration.

You can also just bulk edit the options YAML. (/admin/structure/webform/config/options/manage/time_zones/source)

Or you can use hook_webform_options_alter() or hook_webform_options_WEBFORM_OPTIONS_ID_alter() to dynamic populate options from database or web service.