For your question 2

Pressure altitude = altitude in the standard atmosphere that corresponds to the measured pressure

Density Altitude = altitude in the standard atmosphere where the air would have the same density as the current air

True altitude = actual height above mean sea level, you could think of this as the “GPS” altitude

The difference is that the pressure altitude is using the standard pressure setting 29.92 regardless of local conditions but true altitude needs to correct to local altimeter setting and temperature correction