I honestly can’t say with 100% certainty but I can speak to my experience with playing in an ensemble. For small sets, think a duet like violin and piano, usually there is an agreed upon “lead” and the other player(s) match them. In larger ensembles, band or orchestra, the conductor is god so we should be following them for tempo and queues. That being said, we listen to each other for certain queues and phrases to know when to join in. Generally we match whatever percussion or bass is doing, because that’s usually where time is kept.