Primitive assets in Modeler are designed for creating evenly distributed quad meshes. All primitive creation tools are located on the Add shelf and are also accessible via the Primitives radial menu (Alt+Q).
Unlike Houdini, Modeler primitive tools ignore the Create in Context scene viewer setting. This means that if you are inside a SOP container, primitives will be created directly within it, and the current display node will automatically have a template flag applied.
If you need to quickly merge a new primitive with the existing geometry, you can use the Merge Nodes tool (Alt+M), which combines nodes into a single stream. Similarly, boolean operations can be used. For instance, after creating a primitive in a SOP container, you can immediately subtract it from the current node by running the Subtract tool (-).