It is just like a funeral Mass, but there is no body present, so there is no greeting of the body at the beginning, incensing of the body at the offertory or commendation at the end. Memorial Masses are often offered when the funeral has taken place in a distant location, but there are friends and relatives locally who wish to pray for the deceased, or when there has been a cremation and the ashes are already interred, or when the body has been donated to science. They can also be offered for anniversaries of death.
I think you already nailed it. Sometimes a person grew up in a neighborhood but moved then died in the new area. He or she also got burried in the new area. So those people from the hometown would have the Memorial Mass because the body is not there, so they offer their prayers to the person most of them know.
Well, the basic elements of the Mass will be there. I would think the Homily and Prayers of the Faithful will be more centered around the person who passed away. I’m not sure if any prayers from the Funeral Rite will be included but definitely the Funeral Rite will not be used in full, if at all.
That was the case with my mother. She lived in a retirement community in California for several years, but we took her back east, where much of her family lives, for her funeral and buried her with my father. Then we had a memorial mass at the retirement community for all her friends and neighbors here.
Have also had a couple of cases of people to whom we brought the Eucharist to each Sunday. Their families had all left the Church, and just gave them a quick burial. We had a memorial mass for them. A warning to all to be sure you have your funeral plans in place.