general questions

Was looking into different WP file protection and upload tools.
I have a group of 100 users and need to show them highly secure information.
Therefore proper security measures are important.
In addition those user shall upload a contract, so this process must be possible
(forced, similar name conventions but with difference in user_ID or fields)

I have seen that you are protecting the files on file-level
- is this true, please explain
- if the docuemts are send to google preview, which data security is given
- did you sign any privacy law documents, which are publicly available


I have seen that I can use a ACF to create custom field for each file
questions:
- Can I use any other custom-fields tool too, e.g. Metabox.io
- Let's say one meta-field has content "ABC"
- Can I filter all files using this value (like a loop over all files)
- How I can retrieve the filename or the category-name (via php-function??)

Is there a possibility to get the folder ID of each sub-category of a folder.
Example: folder ABC (10) with sub-folder ABC-1(11) and ABC-2 (12)
Now I need to get an array containing ABC-1, ABC-2, etc.
Using this array I can create shortcodes containing the relevant folders.

I have seen that the user can download files into a specific directory
Questions:
- where the files for e.g. user A1 and user B9 are stored
- if they stored into a single folder
- what happens if user A1 or B9 saves a file named FILE1.pdf
- how I can avoid that all users can see files from other users
- If a logged-in user "A1" uploads a file,
- can I fill the custom fields with defined value (PHP based)
- how this is done?
- which option I have to save such files with specific file name rules
like A1_filedescription_metaboxfieldx_v01 or B9_filedescription_metaboxfieldx_v01 or _v02 (versioning)

Thanks for your first answers.
Most likely I have some more....
Thanks in advance for your ansers.