Columns on the board (stages of work) can be divided into Subcolumns (2nd level columns). These are most often used to detail a stage, or to explicitly divide a stage into 'In Progress' and 'Done' statuses.
For example, the "code" stage may be made up of several more stages. The picture shows how the "code" column has been divided into more detailed stages: "In progress", "REVIEW" and "Ready for testing".
Click on plus sign to the right of column name. Select "Add 2 subcolumns".
The column will then have 2 subcolumns: "Doing" and "Done".
If you need another subcolumn, then click the plus sign next to the existing subcolumn → Create subcolumn → choose a location → Make up a name.
Then you can move the subcolumns to any desired position in the column. To do this press the three dots next to the name of the subcolumn → Move → Choose where you want the subcolumn to go.
If you want to remove one of the subcolumns, tap on the three dots next to the subcolumn name → select Remove from the menu that appears.
You will be given the opportunity to move the cards to another location, or remove the subcolumn together with the cards, before deleting.
If you want to remove all subcolumns and leave just the first level column, click the plus sign on the right of the column name → Manage subcolumns → Merge subcolumns.
If you merge subcolumns, the cards will not disappear, they will all move to the first level column.