How to use joins(inner,left) with xmltable?

I have a xml like below,

<a_level1>
 <level2>
  <list1>
   <a1>123456</a1>
   <list2>
    <variables>
     <a2>115</a2>
     <a3>1313.57</a3>
     <a4>asd</a4>
    </variables>
   </list2>
  </list1>
  <list1>
   <a1>1072001</a1>
   <list2>
    <variables>
     <a2>215</a2>
     <a3>2740.18</a3>
     <a4>qwe</a4>
    </variables>
   </list2>
  </list1>
 </level2>
</a_level1>

I can extract variables from xmltype to table like this.

select a.a1,b.a2,b.a3,b.a4
from x.x_table t,
  xmltable(
    '/a_level1/level2/list1'
    PASSING xmltype(t.xml_data)
    COLUMNS a1 varchar2(20) PATH 'a1',
      list2 XMLType PATH 'list2'
  ) a,
  xmltable(
    '/list2/variables'
    PASSING a.list2
    COLUMNS a2 varchar2(20) PATH 'a2',
      a3 varchar2(20) PATH 'a3',
      a4 varchar2(20) PATH 'a4'
    ) b 
where t.xml_id = 1

result is;

my question is how can i use inner join in this query

You don't need any INNER JOINs, or even the second XMLTABLE, as you can use the full path from the second table and then go back up the hierarchy to get the a1 value:

select a.*
from   /*x.*/x_table t
       CROSS JOIN
       XMLTABLE(
         '/a_level1/level2/list1/list2/variables'
         PASSING xmltype(t.xml_data)
         COLUMNS
           a1 varchar2(20) PATH './../../a1',
           a2 varchar2(20) PATH 'a2',
           a3 varchar2(20) PATH 'a3',
           a4 varchar2(20) PATH 'a4'
       ) a
where  t.xml_id = 1;

Which, for your sample data, outputs:

A1	A2	A3	A4
123456	115	1313.57	asd
1072001	215	2740.18	qwe

db<>fiddle here