WBAMS is extended by modules: an add-on adds a feature and its own pages; a gateway takes money.
Add-ons
An add-on lives in its own folder, declares its settings (WBAMS draws the form), and is activated under Settings › Addon Modules. Once active it can add pages, register hooks, define permissions, create its own tables, and since 1.11.0 offer its own page-builder sections through a cms-sections.json file.
Gateways
A gateway declares its fields, draws the payment step and receives the provider's callback. It must verify every callback with the provider before recording a payment: anyone can post an invoice number.
Retire a module properly
Deactivate it before removing its files. A module deleted while still active breaks the pages that expect it.
Full guide
The complete reference is Addon and gateway modules in the WBAMS documentation.