What usually happens at the end of a probation period at a Korean company?

The most common outcome is that nothing visible happens. Probation in Korea is usually three months, and at a lot of companies it simply ends and your employment continues without anyone mentioning it. Some places do run a formal evaluation with a form and a short meeting, but a manufacturing company in Ansan may well not, so the silence is not a bad sign on its own.

What is worth knowing is that probation is not a free trial for the employer. Once it ends, dismissal requires proper grounds and notice. During probation the standard is lower but still not arbitrary, and the company has to give you a reason. Your contract should also say whether pay changes at the end, because the law allows a reduced rate during the first three months in some cases and quite a few companies apply that, then move you to the full amount afterwards.

The easiest way to raise it is to treat it as a planning question rather than a worry. Catch your team leader after a meeting or at a one to one and say that your probation ends in a particular month, and ask whether there is anything specific you should be working on before then. Phrasing it as wanting to do the job well lands much better than asking whether you will be kept.

Try to get the answer in writing afterwards, even just a short email saying thank you and summarising what they told you. The things worth pinning down are the exact end date, whether there is an evaluation and what it looks at, whether your pay changes, and whether the contract continues automatically or is replaced with a new one. If your pay does change, check the new amount against your original contract when the next payslip arrives.