Mozart is widely known as one of the greatest of composers, yet there is a sense in which his genius is the most elusive and delicate. His greatest compositions hover very close to the trite and banal - and for much of the 19th century he was regarded as a tuneful, decorative but essentially 'light' composer - rather as moderns might regard Telemann, JC or CPE Bach (sons of JS) or Mendelssohn.
It was due to the work of several critical champions such as GB Shaw and Alfred (not Albert) Einstein, and the work of some great conductors, that people began to see the lucid depths of Mozart - and for the late 20th century this was nailed-into-place by Peter Schaffer's Amadeus play and movie, which captured exactly this special quality (albeit marred by excessive plot emphasis on the sub-par and not-wholly-Mozartian Requiem).
Anyway... all this is a prelude to the above tenor aria from Mozart's singspiel opera Die Entführung aus dem Serail (aka 'Seraglio') - The Escape from the Harem.
To appreciate this requires no knowledge of the opera, nor even of the words being sung - indeed, it is a ludicrously long (over six minutes!) and elaborate solo aria for a comic opera, with a complex orchestral introduction - and is therefore nearly always omitted or severely truncated in performance - and indeed in many recordings.
I think it best to regard "Ich baue ganz... (IBG)" as a concert aria - a miniature and self-sufficient solo work of pure music.
IBG is also omitted sometimes, because it is extremely difficult to sing: impossible for most tenors. And indeed so various are its demands on tone, smoothness, agility, range and - especially - breath capacity and control; that no tenor can fulfil all of them to the highest degree.
Tone is especially important for Mozart tenors, as the major roles (and their music) have a special quality of youthful earnestness and purity that should be innate in a singer's vocal quality.
Fritz Wunderlich is regarded as one of the truly great tenors of the twentieth century, and this reputation is rooted in the tonal quality of his voice. Of course not everybody will like it - certainly it is very different from the most popular of great Italian or Spanish tenors (and their South American descendants) - but most agree it is the best suited to Mozart.
But Wunderlich had other qualities as a singer. This aria shows the fluidity of his vocal production, the way his tone continued-between the words, as if the words were being shaped-from a continuous production of lovely sound. This is unusual, and often not even sought-after, among the German tradition of operatic tenors.
Then again his breath control was superb. This is not just a matter of being able to sing long phrases, but of maintaining the quality and control of tone throughout, without diminishment.
His agility was remarkable for someone with such a size of voice; his voice was middle-sized (i.e. middling loud) among tenors; but most tenors who are more fully able to enunciate the florid runs and arpeggios of the middle section of this aria (i.e. separating the notes*) have small/ quiet voices, and without the heroic 'ringing' tone of Wunderlich.
As for the aria IBG itself - well it is a superb example of Mozart at his very best, and doing something only he could do. It is light and easy to enjoy, but has such a glorious sense of spirit and joy in its phrases and touches of orchestration as to reach the sublime.
Pay particular attention to the use of woodwinds in the orchestration, and supporting the voice. Woodwind is regarded as a specialty of Mozart's orchestral work - and here you can see just why.