Yammer is completely separate from SharePoint Online and has different storage.
To clarify. The systems are completely separate. The only commonality is that Yammer Enterprise supports SSO with Office 365.
You would need to look at the Yammer "Network's" analytics to see how much is being stored but there's no real reason to as storage is unlimited. For reference, see the answer to the question
Q. What are the limitations of the solutions (number of users, networks, documents, storage, access, and so on)?
A: Yammer is designed as a SaaS, multitenant environment with scale in mind. The only limit is that of file size. An individual file cannot be larger than 5 GB in Yammer Enterprise (100 MB Yammer Free.) Yammer supports unlimited users, unlimited files, unlimited external networks, and unlimited document types. From: https://support.office.com/en-us/article/Yammer-security-FAQ-Yammer-admin-guide-a2c84111-1da6-4c70-8646-bfe585b93c90
Regarding Teams, they have a SharePoint Team Site behind the scenes to provide the document storage and this is counted in the SharePoint Online storage pool. I'd guess that this site collection would show up in the SPO PS commands but I don't have a tenant handy to check. Also, as of right now, Teams cannot be managed by PowerShell (other than license assignment).