It should be noted that although that video describes Bitcoin and the first version of Ethereum very well, more modern blockchains (very likely including whatever SSB ends up using) no longer use this approach, and the environmental impact has been greatly reduced. Most of the people very upset by this announcement likely don't understand that.
It's a complicated subject, but the simplest explanation for the first generation of "proof-of-work" blockchains is this one: https://www.youtube.com/watch?v=bBC-nXj3Ng4&ab_channel=3Blue1Brown
It should be noted that although that video describes Bitcoin and the first version of Ethereum very well, more modern blockchains (very likely including whatever SSB ends up using) no longer use this approach, and the environmental impact has been greatly reduced. Most of the people very upset by this announcement likely don't understand that.