Casual system explained?

Hey guys, I’m a EU citizen considering migrating to Australia, but I need a little help understanding your working systems.

I have a 10+ yr career in IT/UX, but not sure how the job market in this sector is over there, I‘d assume not so promising for foreigners. However I’m quite interested in your casual system since we don‘t really have sth like that here (at least in my country), which sounds more promising to have a somewhat flexible but plannable income stream rather fast, and I’m anyway looking to get out of IT into sth with a more flexible approach.

Could you explain how this system works in basic terms? You register somewhere and then you get short-term offers to help out various shifts in different companies? Where are those jobs located mostly, in the cities? Do I have to register online, go to some office, do so need certain licenses? Are there certain types of work that pay relatively well while not heavily wearing on the body?

Any advice is appreciated, sorry I guess it’s a rather basic question.