usage is an Azure function for deployment to an Azure Function App.
It will get all the usage data for subscriptions in a management group or billing account and send it to an instance of RCTab.
costmanagement is also an Azure function. It should be deployed to the same function app as the usage function.
monthly_usage uses similar code as usage but runs bi-hourly on the 7th and 8th day of each month to get the previous month's usage.
See the docs for more.