A video that is very "in demand" will distributed it to data centers and "peers" across the world. Peers meaning YouTube/Google makes agreements with ISPs to host popular videos on their servers which are as close as possible the end users, this applies to pretty much all big streaming services.

Streaming doesn't require the server to send the whole video at once, it only needs to send a steady stream of little chunks of the video to each individual viewer every so often.

[deleted]

I feel like I understand streaming better than I ever have before. Thank you for writing this.

Yes a single server can usually server multiple users.

When a video is uploaded, it will usually be saved to one of the machines in the uploaded region. The same machine can now serve many many users. This number will depend on several factors like CPU, RAM, SSD, network bandwidth of the machine…

Such servers can now be distributed across the world and hold a copy of it. This helps reduce buffering on the user side as they will be most likely served by a server in a region closest to them. Ex - server in Oregon handles traffic for folks in the west coast of USA, a server in Dublin Ireland will serve users in Western Europe and so on….

(Algorithms to determine how/when this is needs to copied over)

Depending on the demand in each region can now scale up to the more servers to serve the traffic needs in that region.

(Again determined algorithmically)

On demand videos will be cached locally as well, meaning the file is stored in memory as well as on disk, making streaming to multiple requests even faster. As demand drops, other files will replace the video in the cache.

It’s also worth considering that there isn’t a single version of a video. A service like YouTube or Netflix will create dozens of videos with different codecs and bit rates meaning that someone using a smart tv will watch a different version of the video to someone on a smart phone.

Yes to both questions. The video is copied to multiple drives on multiple computers in multiple locations. But even if it was only on one drive, a single server can serve multiple users simultaneously.