How do you know when you mastered studying a plant in VV1? And does it require only one person to do it, or if mutiple people do it, will they have to start over or continue from the progress of the first villager? Please help, any information would be helpful, thank you

Leave the villager selected and watch for a message in the player communication area. It only takes one successful study to master each plant.

And any number of villagers can carry it out; if you want, you can put one villager on each plant.