If the vocal tract is adjusted to emphasize the harmonic that best projects the voice, the voice is heard well in the hall but might be less well heard up close, especially from the side of the performer. Singers often refer to this as singing "over" or "above" the orchestra or as "floating the tone" out into the hall. A tone, "floated" this way can have great intensity or simply be easily heard or even be a mezzo forte, depending on the demands of the music and orchestra as well as the dramatic intent of that part of the opera.
But it is also possible for the singer to adjust the vocal tract to a harmonic that sounds loudest in close proximity but does not carry well into the hall. If the singer is aware of not being well heard, it is typical for him/her to try to increase the intensity of the sound by excessively increasing the breath pressure. Over time this will do serious damage to the voice. A greater success at being heard in the hall is achieved if the vocal tract is adjusted to a different harmonic, one that projects the tone better
Of course, the ideal is to produce a tone that is heard well in the hall as well as on stage and this can be done with proper adjustments of the vocal tract. But if one must choose between the two, being heard in the hall has to be the major consideration.
Keep in mind that in many musical tones the fundamental pitch of the tone is not as loud as one or more of the harmonics. above the fundamental. The ear hears the louder harmonics best and the brain deduces the weaker fundamental of those harmonics as the understood pitch of the tone.
The range of fundamental pitches of the voice lie well within the loudest range of sounds of the orchestra, somewhere within 300 to 1000 HZ. Consequently few vocal fundamentals are ever intense enough to be heard over the orchestra. It is the greater intensity of harmonics above the fundamental that are best heard by the listeners and their ears and brains translate these harmonics as belonging to the fundamental pitch of the tone.
Over the 350 years in the development of opera, singers learned techniques that allowed them to emphasize harmonics that could be heard over the orchestra and they continued that learning as the opera orchestra grew in size.
It is also true that when a voice is amplified the technique of tuning the vocal tract to the harmonic that will be most easily heard in the hall is no longer necessary and, in fact, such tuning will often overwhelm the microphone and the resultant microphone signal will be distorted. For this reason, singers often sing differently when recording in a studio than when singing in a live performance.