How to dynamically add child elements to an existing element using XSLT 3.0?

I have an XML file containing a main element <root> with a child element <item>. I also have other elements elsewhere in the document that contain information I want to add as new child elements to the existing <item>. Thes elements that i want to transform should have a condition (code="code01"), for other codes the attributes should become new child tags with a prefix __.

here is a simple example of my xml file :

    <root>
      <item>
        <ref>Initial Value</ref>
      </item>
      <extraData>
        <constant code="code01">
          <value>New Value</value>
          <othertags>Random thing</othertags>
        </constant>
        <constant code="code02">
          <value>New Value 2</value>
          <othertags>Random thing 2</othertags>
        </constant>
      </extraData>
      
    </root>

i want to transform it so that it can look like the following :

    <root>
      <item>
        <ref>Initial Value</ref>
        <newTag>New Value</newTag>
      </item>
      <extraData>
        <constant>
          <__code> code02 </__code>
          <value>New Value 2</value>
          <othertags>Random thing 2</othertags>
        </constant>
      </extraData>
      
    </root>

How can i achieve this using an xslt transformation please i'm beginner to this ?

Well, one step at a time.

To get from

<item><ref>Initial Value</ref></item>

to

<item>
   <ref>Initial Value</ref>
   <newTag>New Value</newTag>
</item>

you need a template rule such as

<xsl:template match="item">
   <xsl:copy>
       <xsl:copy-of select="*"/>
       <newItem>
           <xsl:value-of select="//extraData/constant[@code='code01']/value"/>
       </newItem>
   </xsl:copy>
</xsl:template>

To remove the code01 element, you need the template rule:

<xsl:template match="constant[@code='code01']"/>

To convert the other constant elements, you need a rule such as:

<xsl:template match="constant[@code != 'code01']">
   <constant>
      <__code><xsl:value-of select="@code"/></__code>
      <xsl:copy-of select="*"/>
   </constant>
</xsl:template>