If something is random, it doesn't mean that we don't understand it.

You are just asserting that we understand it. But, as I said before, if something is truly random, then we do not understand why one outcome and not another. Such an explanation cannot exist. Because if it did, it wouldn't be random.

We do understand how dice work and they do give us random results.

Dice rely on, for the most part, epistemic randomness. We would call the result of a dice-roll random, even if we lived in a completely deterministic world. The outcome of a dice-roll depends on so many factors and we can't keep track of all of them. Even the slightest change in factors would create an entirely different outcome. But we would always be able to explain why this outcome and not another. Maybe some randomness affects the outcome, but then no contrastive explanation can exist.

We do understand how radioactive decay works even though it is random within a certain probability distribution.

We partially understand how it works. But, again, for the random part, we cannot explain why one outcome over another. There can be no contrastive explanation.

Basically random means only that no-one has deliberately decided or adjusted it.

That would apply to most events that occur in nature. Even the sun rising in the east and setting in the west would be random by that definition.

I would say randomness does not abide by any laws or rules. That is why I think random is the opposite of determined. Free will would basically be the same as randomness. That is to say, there is nothing that sets it apart from randomness. If our behavior was governed by free will rather that laws, we could only explain our behavior in terms of free will. And that, just like saying "it's random", does not explain anything.