If It's Not Baroque, Don't Fix It
Classic music has evolved over the centuries, however, the concerto has remained true to its structure through time. Baroque and classical generally contain three movements: fast, then slow, then fast again. Within the movements, they both alternate the full orchestra with the soloist or soloists. Usually, the full orchestra begins and ends movements, and reappears during the movement in different keys. The difference lies in the formal structure of the key changes and as well as the repetition and development of the piece's theme. The Classical era was steadfast in the formal structures, the sonata and rondo, for example.
While on the subject of concerto structures, the first movement of a Classical concerto will typically be in sonata form. A sonata is a composition for one or two instruments, typically contrasting in form or key. This is where you typically hear the orchestra and soloist banter back and forth while setting the theme for the rest of the piece. Coming second in the concerto is the solo. The solo is written for a single instrument with or without accompaniment. This is where the “rock star” of the concerto gets to strut their stuff with their instrument. Finally, the third movement is called the rondo, which is a composition built on the alternation of a principal recurring theme. Classic rondos also helped to bring an end to the piece lightly, which was different than the first, complex movement.
Concerning melodic distribution, you will find that early classical era concertos would have the whole orchestra present most of the themes and home keys, while the soloist picked up only at the first. One of the common ways to do this in the early Classical era was the "double exposition," in which the full orchestra presented most of the themes, but all in the home key; then the solo entered in the home key, took up the first theme and modulated to the contrasting key for the later...