The bench
A team is a bench, not a line-up. A fixed number sit at the buzzers for a round and the rest wait to be swapped in.
Captain
One seat on the bench is the captain's. On a toss-up it is a seat like any other: whoever buzzes first answers alone, captain or not. On a bonus it is the seat that matters, because the team can talk the question over but the moderator takes one answer, and it is the captain's. Under most rulesets an answer called out by anyone else is simply not the team's answer.
The moderator’s table
A match is run from a table at the front. At a full-size competition the jobs at it are held by different people, so that the person reading is not the person ruling and the person timing is not the person scoring.
Moderator
Reads every question aloud, recognises whoever buzzed first, and takes the answer. The moderator's reading sets the pace of the round.
Judge
Rules on whether an answer is right, including an answer worded differently from the one on the sheet, and on anything else the rules leave to a call.
Timekeeper
Runs the answer window after a buzz, the conferring window on a bonus and the clock on the round itself, and calls time when each one closes.
Scorekeeper
Keeps the running score for both teams and reads it out at the breaks, so that both benches can check it before play goes on.
Recognition
“A buzz is not an answer.”
A buzz is not an answer. When a lamp lights, the moderator names that seat, and only then may the player speak. An answer given before being named is a blurt, and under most rulesets it is treated as wrong whatever was said. Conferring is the same: on a toss-up, a word or a signal to a teammate costs the team the question.
You may buzz before the moderator has finished reading. Get an interrupt right and it counts the same as any other answer; get it wrong and most rulesets charge a penalty on top of the lost question, which is what makes interrupting a decision rather than a reflex.
Substitutions
The bench exists so that players can be swapped. A substitution happens at a break — between rounds, or at a set point inside one — and never in the middle of a question, and the moderator's table is told who is coming off and who is going on. Which breaks allow it, and how many players may sit out and come back, is set by the competition.