The Mass begins with the Introductory Rites and the first part of this is the Entrance.
Near the back of the Roman Missal there are four prayers under the heading “Preparation for Mass”. They are not part of it, they are preparation for it.
In the Ceremonial of Bishops, Chapter 1 it describes the Stational Mass of the Diocesan Bishop. Under the heading “Arrival and Preparation of the Bishop” it has things like the bishop washing his hands, putting on vestments, a deacon placing a miter on his head, the bishop putting incense in the censer and blessing it with the sign of the cross. Then under the heading “Introductory Rites” it has:
“128 As the entrance song is being sung, the procession moves from the vesting room (sacristy) to the sanctuary (chancel) in the following order:
– censerbearer carrying a censer with burning incense
– an acolyte carrying the cross …”.
Perhaps the blessing of the incense is a ceremony in preparation for the Mass, which begins with the Entrance, which begins with the first person in the procession leaving vesting room (sacristy).
Another approach would be posture of the congregation. The General Instruction of the Roman Missal (GIRM) has “43. The faithful should stand from the beginning of the Entrance Chant, or while the Priest approaches the altar, until the end of the Collect.” On this basis the Mass begins when the congregation are standing.
How do people know when to stand? In some places a bell is rung. Perhaps they see the entrance procession. Perhaps there is an announcement made like “Please stand and join in singing hymn number 44.” Perhaps this comes after a longer announcement like “Please make sure your mobile phones are on silent.”
Are these announcements part of the Mass or a preparation for the Mass? I would say a preparation. The Roman Missal does not describe the Mass as beginning with such an announcement. The GIRM has: “50. … After the greeting of the people, the Priest, or the Deacon, or a lay minister may very briefly introduce the faithful to the Mass of the day.” Obviously this introduction, at this time, is part of the Mass.