Line of sight and unit damage

New to 40k and trying to understand true line of sight, I understand that if one model is visible then the unit is a visible unit.

So on the back of that does this mean I can allocate wounds to any model that is also not visible at all in that same unit.

Another vague area is if only 1 model is visible in a unit of 10 legionairres, I take wounds and give it to the visible model, that model is then destroyed, after removing the visible model from play, can my opponent continue to shoot the unit of 9 remaining legionairres that was classified as visible or is my unit now out of line of sight and can no longer be attacked?

TIA