Add a new column from value that is stored in a list of cell of the table

I'm quite new to PowerBI development, and I need your help.

I have a table build from a json:

And I want to add information to this table, using data from the column "fille".

This cells are fill with 'List'.

This 'List' are 'Record' :

And inside this record, I can find my information:

The field that I need is :

For each row of main table
    set the value "avancement" of the first 'Record' of the 'List'

How can I reach to do that ?

I'm trying something like this, without success :

tableDev = Table.AddColumn(tableInfoFiltree, "Dev", each tableInfoFiltree[filles]{_}{0}[nom], type text)

I try also to build a function, but, the same, I don't reach to extract the List from the cell to put it in parameter.

Thanks for the help

Try this.

= Table.AddColumn(tableInfoFiltree, "Custom", each [filles]{0}[avancement])