[ehnto]

Edit: sorry, you got this question a bunch below. I didn't read ahead.

Are probation periods not a thing in the US? Maybe because of at-will termination?

In Australia most roles have a 3-6 month probation period in which either party can terminate the employment agreement at will. That's essentially a "paid trial period".

After that, employers have much stricter rules for what reasons they can fire you (in theory) and and both employers and employees must give 2 weeks notice or extra if agreed upon in contract.