I understand why it is necessary to maintain something, but I am curious about what websites do to maintain themselves. Does it mean it is adding new features, fixing things, or just a reboot? Also, what is Scratch doing in this maintenance? Are they fixing cloud variables yet? I am very curious about this!

Maintenance just means to fix something to preserve it. This is just some updates to fix comments.

That depends entirely on the site and how it works on the backend. This is just routine maintenance that happens every few years or so, nothing special. Cloud variables are run by a separate set of servers from the main site, and will be fixed when they get around to fixing them.