How does QAM retain the phase of the signals?
One method: -

You have to transmit something in the data that can be unambiguously recognised as a sync. You look to decode this in the receiver then you have acquired sync. It doesn't need to be transmitted very often of course but, until you receive it, you are out of sync. The payload data transmitted must be configured so that it doesn't accidentally transmit the sync.

Another method transmits a marker bit that is different to regular data. This of course means establishing what the relatively slow moving but ever changing ebb and flow of amplitude information is, and looking for a bit that has an amplitude that is clearly different to regular bit amplitudes.