It is quite typical when you need to read data from and write data to the specific subfolder within the same base folder. You can use the same connection to read files from (and writes files to) the specific subfolder under the same folder.
For example, if there is a folder /opt/data which contains subfolders:
and the Server storage connection points to the directory
you can use that connection together with a relative path in the FROM (also TO).
/03012020/*.csv will read all files in