As far as single stream, Disney's Hotstar claimed 59 million for last year's Cricket World Cup, and as far as the YT platform, the Chandrayaan-3 lunar landing hit 8 million.

100 million is a lot of streams, let alone 300. But also note that not every stream reaches a single individual.

And, as far as the 59 million concurrent streams in India, the bitrate was probably very low (I'd wager no more than 720p on average, possibly even 480p in many cases). It's again a very different problem across the board due to regional differences (such as spread of devices, quality of network, even behavioral differences).

The world cup final itself (and other major events) is distributed from the host broadcaster to either on site at the IBC or at major exchange points.

When I've done major events of that magnitude there's usually a backup scanner and even a tertiary backup. Obviously feeds get sent via all manner - the international feed for example may be handed off at an exchange point, but the reserve is likely available on satelite for people to downlink on. If the scanner goes (fire etc), then at least some camera/sound feeds can be switched direct to these points, on some occasions there's a full backup scanner too.

Short of events that take out the venue itself, I can't think of a plausible scenario which would cause the generation or distribution of the broadcast to break on a global basis.

I don't work for OBS/HBS/etc but I can't imagine they are any worse than other broadcast professionals.

The IT part of this stuff is pretty trivial nowadays, even the complex parts like the 2110 networks in the scanner tend to be commoditised and treated as you'd treat any other single system.

The most technically challenging part is unicast streaming to millions of people at low latency (DASH etc). I wouldn't expect an enormous architectural difference between a system that can broadcast to 10 million or 100 million though.