Hi,
Helpers are being trained before they become Moderators. The promotion depends on the progress of the Helper.
As for Sr. Mod, a Moderator achieves such promotion after having great interactions with the community, handles their Moderation responsibilities very well, is very experienced with the things a Moderator is required to do and/or takes on more responsibilities.
If you have any other questions, feel free to ask staff or a helpful community member :)