How is the coordinate system working in URDF

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I am a beginner and I can't seem to understand as I was going through some examples of URDF that how the Coordinate system is working here though, I am sure it's working normally but it's not making any sense to me. Below is an example.

<?xml version="1.0">
<robot name = "my_first">

 <link name="base_link">
  <visual>
    <geometry>
     <cylinder length = "0.01" radius="0.2"/>
      </geometry>
      <origin rpy = "0 0 0" xyz = "0 0 0"/>
      <material name = "yellow">
        <color rgba= "1 1 0 1"/>
      <material/>
     </visual>
    </link>

    <joint name = "pan_joint" type = "revolute">
     <parent link = "base_link"/> 
     <child link = "pan_link"/>
     <origin xyz = "0 0 0.1"/>
     <axis xyz = "0 0 1"/>
    <joint/>

   <link name = "pan_link">
    <visual>
     <geometry>
     <cylinder length = "0.4" radius = "0.04"/>
     <geometry/>
     <origin rpy = " 0 0 0" xyz = " 0 0 0.09"/>
      <material name="red"> 
        <color rgba="0 0 1 1"/> 
      <material/>
    <visual/>
   <link/>


    <joint name="tilt_joint" type="revolute">  
       <parent link="pan_link"/>  
        <child link="tilt_link"/>  
         <origin xyz="0 0 0.2"/>  
         <axis xyz="0 1 0" /> 
    </joint>


 <link name="tilt_link">    
   <visual>      
    <geometry> 
      <cylinder length="0.4" radius="0.04"/>   
    </geometry>     
 <origin rpy="0 1.5 0" xyz="0 0 0"/>
      <material name="green">
        <color rgba="1 0 0 1"/>
      </material>
    </visual>
  </link>
 </robot>

How are the Pan_link's and tilt's xyz is calculated ? Whats the differnce b/w origin rpy and xyz ? please help me out here