In the Layers and Objects dialog, [right-click] the layer you want to move and [Layer to Group]. Still in the dialog, [right-click] the resulting group and [Move to Layer].
You can also drag and drop items in the dialog to make one layer a sublayer of another. The GUI hints where the dragged layer is going to land in the document structure, either above or below or sublayer of another layer.
Beginners' Questions Uniting Layers
Inkscape.org
Is it possible to unite contents of 2 layers to 1 layer?
Yes, but not in a single step.
In the Layers and Objects dialog, [right-click] the layer you want to move and [Layer to Group]. Still in the dialog, [right-click] the resulting group and [Move to Layer].
You can also drag and drop items in the dialog to make one layer a sublayer of another. The GUI hints where the dragged layer is going to land in the document structure, either above or below or sublayer of another layer.