QAM Amplitude Levels
n real life how does communicating wireless devices know which amplitude levels to agree on for digitally modulating-demodulating bitstreams?

You can't &quot;agree&quot; on levels, since the received amplitude depends on the attenuation on the channel, and that depends on distance and a lot of other factors.
So, your receiver depends on detecting something that it knows the right amplitude (or power) of – for example, a preamble, or an average squared symbol, or something like that.
How that is done depends on the system you look at – generally, this is part of the synchronization and equalization efforts every system has to make. And it really differs! For example, some continuously transmitting system will not insert any specific symbols for that, but depend on the receiver doing statistics. Others have a &quot;silence&quot; period followed by known symbols; other systems are packetized and depend on preambles with specific autocorrelation properties (but not necessarily 100% defined content). Other systems depend on a pilot tone, and there's many more options.