In between software jobs I worked (part time) as an admin assistant for our parish’s director(s) of Liturgy and Music. At various times the ‘positions’ have been separate or combined, paid or volunteer.
At least when I worked there the director(s) chose the songs for the Sunday, Holy Day, and special occasion Masses, scheduled/hired choirs, cantors, and instrumentalists, and coordinated the various ‘ministries’ (readers, sacristans/EMHCs, ushers, greeters, altar servers, wedding coordinators, and decorators). The director(s) also worked with the RE/Faith Formation groups to coordinate baptisms, first communions, special group liturgies, and RCIA activities. (Many such groups have very fanciful ideas for liturgies and must be gently but firmly brought back to reality.)
The liturgy/music director has to be informed on what is in the GIRM and how it is to be interpreted if there is any confusion, and advise the pastor. He/she needs to keep in contact with the diocesan liturgy head in case there are any special diocesan regulations.
In our parish the liturgy director would usually recommend the Mass options to be used at the various Masses. (Yes, the priest was free to make his own choices; but most priests were very happy to have been freed from making even such minor decisions.)
The liturgy director would come up with a detailed Mass order of service (think of a a kind of script) that covered all the Mass options which would be used, including prayers, any special activities for that Mass, the prayers of the Faithful for that Mass, and special blessings, and the like. This was used in lieu of the Ordo. (Actually, much of it came from the Ordo.) A copy of this went to the presiding priest, the reader in charge of the prayers of the faithful, and the musicians. A spare went in the sacristy for anyone else who might need to know. We had binders for the priest’s copies (available from liturgical supply companies for those who use electronic reactionaries/ordos.)
Visiting priests loved us because they always knew just what to expect.
When I worked for the parish my jobs were to see that new music went into the choir members folders, ordering new music and inventorying it when it arrived, creating sign in sheets for the Sunday Masses for the clergy, altar servers, readers, EMHCs, and CLoW team members, getting the Mass orders of service to the proper places, assisting the deacons with music choices for funerals, sending meeting notification messages, mailing/emailing schedules to the ushers, EMHCs and altar servers, helping various groups who needed to practice in the church find available time, and the usual sheltering the director(s) from those ministry leaders who were demanding more time from the director than their ministry warranted.