I believe it is 72 hours before the leg that checks your Thai pass etc. And of course, any transit would have to be on the same ticket in that scenario, and airside. I think different airlines have different processes as well. If your layover time is great enough, you may be required to pick up your transit ticket at the transit/transfer desk, and they may check all the paperwork then. I have no first hand knowledge of this though.
I actually don't know about length of stays etc, because I am visa exempt, and usually don't do Thailand for more than 3 weeks at a time before flying somewhere else anyway.
The hotel needs to be pre paid, and the hotel needs to send you a receipt showing payment. The PCR tests need to be prepaid. Your I surance needs to be pre paid.
All this has to be complete before starting the Thai Pass process.
Quarantine in the sense that you have to stay in your room until you get your airport negative test, and after that, you have to stay on Phuket for 7 days and your day
, I know. It is a very loose type of quarantine. But it is technically a quarantine, as it restricts movement (off island) to help contain and control.
You have to book and pay for your Sha+ hotel, your PCR tests, and your insurance before you do the thai pass. You also have to input a Flight date and number (Although you aren't required to prove that you paid for the flight yet.)