In Australia you can have as many jobs as you want, zero issues. id be appalled if I needed my employers permission, like a little kid asking their dad for permission.

It depends on the nature of the job.

If you are a educated professional, then companies get pissy about how your second job might interfere with your primary work and erode some competitive advantage.

If you are working hours in a fast food place, they don't give a shit unless you fail to cover your shifts and never are available to pick up a shift for someone who can't cover theirs.

Many countries, sure. It would not be true for most countries. Not to mention freelance work, consulting gigs, or general self employment on the side.

How common it is may be different than whether or not it is allowed.