you need to write vba code sth like this:

Createobject FileSystemObject say fso.

get file name using GetFileName method of fso

iterate through the returned file names and assign it to the column B cell

Alternatively u can just use Dir(path) function similarly without creating fso object.

U need to put this code in the change event of the worksheet.
Also to prevent any other change in the worksheet to trigger above action,
do a intersection check of the changed range and the target range of cells.