SAP MM is nothing but a submodule within the SAP ERP solution itself ( and this module deals with procurement processes within the ERP and integrates with the rest of the modules like SAP FI, CO, PS SD and so on).
If you want to learn SAP MM from scratch, then here's the right video for you:
Whereas, SAP SRM is a separate solution component that is entirely different from the SAP ERP and which majorly solves the procurement in an organization by adding cataloging details and giving deeper insisting into issues like spend analytics particularly for Supplier interaction and management. Since SAP SRM has its dependency on SAP ERP implementation. So, if you have to implement SAP SRM, then you will at least need to procure the SAP ERP license as a base foundation to get started with.
On the other hand, SAP Ariba is just like the Alibaba/Ali Express/Indiamart (marketplace) of SAP procurement, and its a closed ecosystem for buying and selling where SAP is used to manage the close interaction between buyers and sellers, it also processes invoices, etc. and makes life easier for both the buyers and sellers in the ecosystem.