Something can’t be determined if no one knows the answer.

Yes it can. The trajectory of the planets is well determined and can be predicted many years into the future. That was also true before astronomy, or before humans even. Even if nobody currently knows how to predict something, it can certainly be obeying deterministic rules.

Why do we assume determinism removes our free will? If I were a parent you could bet with almost certainty that I would take a bullet for my child, does that mean it’s not my choice because it’s predictable?

Under determinism there is no such thing as choice. Not whether you take a bullet for your child, not whether you watch TV or read a book, not whether you look at the wall for ten or twenty seconds. If there can be no choice, there can be no free will.

Why do we assume randomness gives or allows for free will?

The proposition is not than random behaviour is free will. The proposition is that since determinism precludes free will, the existence of free will must require a non-deterministic world. This does mean we know anything about how free will works or comes about, and certainly it doesn’t mean that non-determinism implies free will. We can definitely be living in a non-deterministic world without free will.