A niche is a recess in a wall which often contains a statue. A church could certainly have a niche that holds a crucifix. A niche need not be in a church.
An apse is a large recess at one end of a church. If often contains an altar or statuary and is its walls are covered by paintings or carvings or other artwork. A large crucifix is a possible central figure for an apse. An apse is a term for a part of a church but it need not hold a crucifix.
Another name for a church recess would be apse.
Apse
Another name for a church recess is apse. They are typically placed at the eastern end of the church and contain the altar.
An absis is a recess or projection at the east end of a church - another name for an apse or apsis.
Sacraments are not things that are 'kept' in the church. They are religious ceremonies or acts of the Catholic Church that are regarded as an outward and visible sign of divine grace. If you are referring to the Eucharist, the Blessed Sacrament, it is kept in the tabernacle.
I don't think there is a special name for it, so I suggest you just call it "a large crucifix".
it's called a crucifix
The crucifix is a cross with Jesus on it, the cross is simply two perpendicular planks of wood.
It is usually kept in the alter. Another name for is the holy oil.
The place where the host is kept in church is called the Tabernacle. It is a locked box or cabinet where consecrated hosts (the Eucharist) are stored for adoration, communion, and to be taken to the sick.
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Alcove