Does Kleros draw between online jurors, only? If yes, how it detects who is online? And if no, how it recognize drawn juror is absent and another juror should be drawn?
Kleros draws tokens, not jurors.
If you want to be a juror, you stake a number of tokens to a specific court. Each time that respective court receives an arbitration request, it draws randomly among staked tokens. If you own any drawn tokens, it means you have been drawn as a juror.
This means the chances of getting drawn are in a linear relationship with how many tokens you have staked.
If you are drawn but do not cast a vote before the deadline, you get a penalty. You lose your “locked” tokens. The amount of locked tokens per vote varies between courts.
If you fail to vote, you also automatically get unstaked to protect you from losing your tokens overtime due to inactivity.