As she mentions, Bach was the first composer of what we might call a keyboard concerto and he did it by first transcribing a number of violin concertos by Vivaldi for one or more keyboards and orchestra. In 1721 he gathered together six of his original concertos, including one for keyboard soloist (though joined by solo violin and flute) and sent them to the Margrave of Brandenburg in hopes of winning a court appointment. Alas, the Margrave did not even send an acknowledgement and they were likely never performed. The six together, each for a unique group of soloists, are now known as the Brandenburg Concertos, after the Vivaldi Four Seasons, probably the most famous concertos from the Baroque era. Richard Taruskin in the Oxford History of Western Music makes a special point of how very eccentric these concertos are. Each group of soloists is an oddity in itself. The first concerto, for example, has the bizarre combination of two natural horns, three oboes, bassoon and violino piccolo, a small violin, now obsolete. These are the solo instruments! Let's have a listen. Here is the Freiburg Baroque Orchestra with that first concerto, in F major. The piece begins at the 40 second mark:
That's quite a sound! One wonders what Vivaldi would have thought. Equally unprecedented is the role of the harpsichord in the Brandenburg No. 5, where, halfway through the first movement, about where an Italian concerto would have been coming to an end, the harpsichordist takes over and launches into a lengthy solo that is unique in the Baroque era. Akin to what later on became the soloist's cadenza in the Classical era, at the time this was unprecedented. The harpsichord solo starts just after the 6:30 mark in this recording by the Cleveland Baroque Orchestra.
Bear in mind that a whole concerto, in three movements, by Vivaldi only runs around ten minutes in length. The Brandenburg No. 5 first movement alone is over ten minutes.
Bach also transcribed his own original violin concertos. One of the most famous of his concertos is the one for two violins in D minor, that he later transcribed for two harpsichords in C minor. Here is the original for two violins: