The first thing you see there is this.
The standard is 0dbm which is 1 mw with a 600 ohm load. Or as stated above 774.6 mv drop across a 600 ohm load. If all our measurements were across a 600 ohm load it would be simple to use a meter and read dbm direct. When the load changes the game changes. Now let's look at the stage characteristics.
They say current gain is infinite! Assuming the input current was zero it would be. In the real world we have some current so we will look at that more later.
The voltage gain is very near 1. So we are told the current gain is limited by the load current at a voltage level close to the input. Now let's look at the circuit.
What if the output was 50 ohm?
So what have I learned from this exercise? Let's look at a circuit with dual output and see.
In this sample circuit I am using the JFET as an electronic transformer to match the load. My load can be either 60 ohm earbud or 2k ohm headset. What would the output be?
I could optimize the circuit and make it more efficient for either purpose but it would drive either an earbud or headphone as is. (The sim is using -80dbm drive)
Hopefully that makes since and you can see the terms are relative and you need to know the point of reference to use the numbers.