The file used in the manipulation tutorial uses a xacro file that doesn’t exist in the real repo. (https://github.com/UniversalRobots/Universal_Robots_ROS2_Description/tree/humble/urdf) How was it generated? It looks like the following was added, but without explanation of why.
<link name="ground_plane_box">
<visual>
<origin xyz="0 0 0" rpy="0 0 0"/>
<geometry>
<box size="1.0 1.0 0.5"/>
</geometry>
<material name="Grey">
<color rgba="0.7 0.7 0.7 1.0"/>
</material>
</visual>
<collision>
<origin xyz="0 0 0" rpy="0 0 0"/>
<geometry>
<box size="1.0 1.0 0.5"/>
</geometry>
</collision>
</link>
<joint name="ground_plane_box" type="fixed">
<parent link="world" />
<child link="ground_plane_box" />
<origin xyz="0 0 -0.25" rpy="0 0 0" />
</joint>