Who worked at the medieval church?

Clearly, the priests did.

Peasants provided services ranging from carving pews to cleaning up. Whether this was a part of tithing, I don't know.

Certain individuals were hired by larger churches to be sextons.

Monks and nuns attended churches with everyone else (or perhaps everyone else attended with them) in some places, and any group might be the choir.

Some churches had organs, and organists were often paid to play. Other Musical Instruments were also used, and musicians were often present.

Grave diggers were often hired, as were people to tend churchyards and church gardens.

During construction and repairs, professional carpenters, stained glass makers, and so on were employed.

In some places there were special architectural features, such as the rood screens in Britain. These were carved and decorated by artists, who put paintings on them. Also, in many places sculpture was put in churches, so sculptors were employed.

Many churches had important things that were made off site. These included bells and bibles. The former would have been made in shops where bells and cannons were cast, and the later at monasteries.

Some things happened at churches that were not canonical, though they had religious basis. These included the production of morality plays, which were intended to educate as well as entertain.

Other things happened at churches, as for example Archery matches. Some churches were even built near yew trees so the peasants could get a supply of wood for bows and arrows from the church. I do not know if there was any employment associated with either.