I think most answered your question with one exception. Dollars? What else you expect it in? Cats?

(Ignoring you will want it in the country of your residence)

Generally the people you’re telling your salary are people in the same location as you or at least the same state.

Telling the yearly salary makes the most sense there since it should be similar weekly/monthly amounts. And even if they aren’t in the same place and it doesn’t translate based on tax differences, telling someone the monthly or weekly payment would make no sense.

Many US jobs are based off an hourly rate, some with overtime (usually not added in). I noticed other posters mentioned Xmas bonus. As an hourly worker I received a standard 3% yearly raise to cover increased costs of living. Because our cost of living increase was nearly double that, our salary actually declined. Oh and that Xmas bonus... If you count a 25$ gift card to Walmart a bonus...

Pretty shit, but it could be worse.

It's just a choice. It means nothing. Conventions are conventions merely because people started doing it that way. If you don't understand, then ask a question.

What exactly is your challenge here?

In Australia we mostly get paid weekly.