How to configure storing to log file / text database?

To have a log file / text database on your server, you need to do:

Create a log file template (e.g. "logtemplate.txt") and place it on your server (for example in your form folder). The first line of this template would be a file name where data will be written (it could be a local path to navigate out of the directory where the template is).
The rest of the lines are the template, which is filled with data on every submission and then appended to the log. Please include a line break in the bottom line of the log template, so each new log entry will be appended below the existing records and not on the same string:
Example (single-line CSV):

Another example -- multi-string log template (in the current folder):