I am trying to connect multiple data sources. But tableau is making it to inner join, how to make it full outer join

You can change the type of join from the data pane itself. Means the window where it is showing you the inner join, just click on the join icon and you’ll be able to see the options for all the joins-

1. Inner join
   
2. Full outer join
   
3. Right outer join
   
4. Left outer join
   

From here, you can select the right kind of join and also can select the joining column on which you want to implement the join.