What is a ritornello form?

The ritornello form was one of the musical structures developed in the Baroque period. It is characterised by a recurring A section in between new sections of music, and is often described as 'ABACA', where the A section contains a distinctive theme. Importantly, the recurring A section is rarely an identical repeat of the first time we hear it. It often returns in keys related to the tonic, such as the dominant or a relative minor, and if the original iteration was very long, it returns in a shortened form. The sections in between the ritornello sections (B or C) are called episodes.

An excellent example of a piece that uses a ritornello form is Vivaldi's Bassoon Concerto in E Minor: https://www.youtube.com/watch?v=YWlsLJlpyPU. The first A section opens the piece, with the first episode beginning at 0:51, when bassoon solo begins. Notice how the bassoon solo line uses the melody from the A section, but think about why this is not an example of the A section returning. The next interation of the ritornello theme/A section can be heard at 1:39, when the full orchestra returns. How is this different from the first time we heard it? Is it in the same key? The second episode begins at 1:52, and so on.