Where are Stream files stored for Enterprise O365 Tenants?

The data is stored in its own storage based on Azure, specifically blob and also uses SQL and Media Services - see articles here

There is no current way to access this directly, only via the Stream portal.

Stream quotas are separate from SPO quotas as also confirmed in these articles

https://docs.microsoft.com/en-us/stream/faq#which-regions-does-microsoft-stream-host-my-data-in

https://techcommunity.microsoft.com/t5/Microsoft-Stream-Archive/Learn-how-Microsoft-Stream-was-built-using-Azure/ba-p/285185

https://techcommunity.microsoft.com/t5/Microsoft-Stream-Forum/Stream-Storage/td-p/79182

The search has a uservoice open for this

https://techcommunity.microsoft.com/t5/Microsoft-Stream-Ideas/Ability-to-add-metadata-and-being-able-to-sort-filter-search-by/idi-p/87488

There is a uservoice open for importing videos to Stream from SharePoint

https://techcommunity.microsoft.com/t5/Microsoft-Stream-Ideas/Upload-Video-from-Sharepoint/idi-p/128974

Hope that answers your questions! And confirms your suspicions are correct!

Best, Chris