Is "First Shot" "Second Shot" and "Booster" the same as 3 "doses"?

According to the CVS person who did my booster, the booster for Pfizer is a full dose while the Moderna booster is a half dose (relative to each manufacturer's original first and second shot doses).

According to this source https://www.statnews.com/2021/02/02/comparing-the-covid-19-v... the original Pfizer dose was 30 micrograms of vaccine, while Moderna's was 100 micrograms.

If that is correct, then Moderna's booster would still be more vaccine (50 micrograms) than Pfizer's original dose.

Of course more is not necessarily better!

Thinking seriously about this, I will ask, were these different dosages a work of collusion between these companies and the government because they were so unsure of the reaction and efficacy at a specific dose? Was it, in effect, a huge "clinical trial"?

Between Pfizer and Moderna what is the difference other than dose? Didn't they use the same mRNA? So what was the science behind them choosing different doses? Were they guessing or were they colluding?

If someone could answer my question with a logical response instead of just down voting it I would appreciate it because it is a serious question I am trying to understand about the vaccines. I am debating on getting a booster but I have a risk profile that makes this more of an issue for me.

So, what was the science behind the different doses? Does anyone know if Pfizer and Moderna used different mRNA?

The different doses were guesses made to balance effectiveness with severity of side effects. (Earlier Moderna trials, at least, had included even higher doses, which could cause pretty severe side effects.) Their choices had to be locked in early, based on very limited preliminary trial data, in order to run their phase 3 trials as quickly as possible, to actually bring the vaccines to market quickly in 2020. They just made somewhat different bets. I think it's as simple as that.