Depends on a multitude of factors, like how urgently the position needs to be filled, that particular HR department's schedule, and so on. For example, if I'm one of the people in charge of that job listing, and I happen to have an empty schedule for the time being, I might take this chance to start clearing some of the candidates on my list rather than let it pile up in the future.
Exactly what it says. It means the HR for that posting has started processing the applications.
For most
SBs at least, if you're going to be selected for an interview, we'll call you.
Failing that - we'll call you again.
If it still doesn't get through, we'll text you, or drop an email, or both.
If you went through C@G, you might not get a rejection email. We might just tick off as "Unsuccessful" on C@G.