Q. 1. When I make a donation (Mass stipend) to have a Holy Mass celebrated, where does that money go?
A. 1. When you make a donation as a Mass Stipend to have a Mass celebrated on your behalf, that money goes to the priest who celebrates the Holy Mass.
In some countries like in India, the priestly income from the Diocese is so small that the Mass Stipend donation supplements their income so they have a little more to spend on their basic needs, clothing, food, personal hygiene, travelling, supporting the needy, etc...
As no one tells you how to spend your wage, you should not be questioning on how the priest spends his minimal income at the service of the Lord Jesus.
The priest is only allowed to accept one Mass Stipend per day. So he is not getting rich on that small income. If he has received too many Mass Stipends, he must pass them on to the Diocesan Office so they can be distributed to other priests or missions.
For many priests in poor countries, a Mass Stipend, although not mandatory to have a Holy Mass celebrated, is a gift from Heaven because of their extreme poverty.