Video is saved as a binary blob in some kind of storage. In our case that would be IndexedDB. But since the data is uploaded in parallel with the call there is only few hundred MB used at any give moment.

Generally browsers will let you store a few GB of data (depending on the user free space) if your really wanted to but it's not recommended since they can delete it any time.

We store unuploaded chunks of the video in IndexDB in a browser. Btw if plan to do something similar beware of the fact that storing a lot of blob references in IndexDB can cause some wired issues in Chrome a some blobs can get corrupted. Our workaround is to store it as byte arrays.