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No, $dV$ is not always equal to $dx \, dy \, dz$. In Cartesian coordinates, $dV = dx \, dy \, dz$, but in other coordinate systems, such as cylindrical or spherical coordinates, the expression for $dV$ changes to account for the geometry of the coordinate grid.

Conditions: The current setup uses rectangular/Cartesian coordinates.; In other coordinate systems, such as polar-type coordinates mentioned verbally, $dV$ may take a different form.

No, $dV$ is not always equal to $dx \, dy \, dz$. In Cartesian coordinates, $dV = dx \, dy \, dz$, but in other coordinate systems, such as cylindrical or spherical coordinates, the expression for $dV$ changes to account for the geometry of the coordinate grid.

Conditions: The current setup uses rectangular/Cartesian coordinates.; In other coordinate systems, such as polar-type coordinates mentioned verbally, $dV$ may take a different form.

The differential mass element $dmass$ is derived by multiplying the local density $\rho(x,y,z)$ by the infinitesimal volume element $dV$. For the example density $\rho(x,y,z)=xyz$, this gives $dmass = xyz \, dV$.

Conditions: The object has non-uniform density.; The density is modeled as a function of position $(x,y,z)$.; $dV$ denotes an infinitesimal volume element.

The differential mass element $dmass$ is derived by multiplying the local density $\rho(x,y,z)$ by the infinitesimal volume element $dV$. For the example density $\rho(x,y,z)=xyz$, this gives $dmass = xyz \, dV$.

Conditions: The object has non-uniform density.; The density is modeled as a function of position $(x,y,z)$.; $dV$ denotes an infinitesimal volume element.