A concerto has a group of soloists rather than just one.
A concerto grosso has a small group of soloists whereas a solo concerto has only one
Yes, A blank grosso has a group of soloists rather than just one
A concertino is a solo group in a concerto grosso.
A concertino, or small group of soloists (usually 2 violins, a harpsichord, and a cello), contrasts with the cocerto grosso, which is the whole string orchestra and its ripieno players.
In a solo concerto there is only one person playing in contrast to the orchestra, concerto grosso involves a small group of musicians playing in contrast to the orchestra.