With regards to relief by units, we definitely know it was done.Except we don't really know what those sources talk about. These are all just interpretations. I have posted video above where whole thing is been discussed from the point of our current knowledge. Nobody knows how it was done and if it was done at all to begin with, that is when Tacitus and Polybius talk about it, we don't know what exactly they talk about.
The fighting took place in this spot, which was unfavourable both from its confined limits and because they had halted just under the very spurs of the mountain, so that no missile failed to reach them. Nevertheless they strove with valour and endurance and sustained every description of wound. The forces of the foe were increasing and cohorts were continually being sent up to them from the camp through the town so that the unexhausted were always taking the place of the exhausted. Caesar was obliged to adopt the same course of withdrawing the exhausted and sending up supporting cohorts to the same place.
(from Caesar's Commentaries)
That is perfectly clear in meaning. I don't know how anyone could look at that and think there is ambiguity there. You can certainly say Caesar is lying immediately raises the question -- why?
Nobody serious has ever assumed battles look anything like they do in any television show ever aired. The stuff in that scene from HBO's Rome was apparently made-up by one of the technical advisers. Obviously that carries over into games with action elements, like Bannerlord. A game where you basically engage in a low-density series of duels and small skirmishes while darting back to the safety of your formation for long periods to rest up would not be well-received.When looked at the data we have from another angle, it's clear that combat must have been way less intensive then we usually assume.