A few questions that need answers!

1 (screenshot) version 8.0.2 clean installation, version 4.4.0 with a working site. The owner and the group are different, do I need to make a change to version 8.0.2 and rewrite all folders and files to www-root? Or leave it as it is?
File config.php it has the attribute 666 everywhere, in version 4.4.0 the site works well,
does it need to be redone to 644 in version 8.0.2?

2 (screen) version 8.0.2, In Configuration information, Folders those are not writable: SSH file, but the attribute 755 is worth it.
If you redo the attribute to 777, then everything becomes good, why?

3 (screen) Version 8.0.2 The attribute of the folder oc-content/uploads/temp 777, if you redo it to 755, the photos will NOT be uploaded to the goods. Why?
And watermark does not want to load, writes an error, added it manually, did not help.

4 (screen) Maybe it will help for answers somehow?

And yet, both servers have PHP CGI mode of operation,  PHP (CGI) version 7.4.29 (alt).
There is no way to install Apache, this is the structure of the server, but the 4.4.0 site works fine.
Will it fully work on PHP CGI, 8.0.2?

Try to answer all the questions point by point, thank you!

Here is the error;  WWW-запросов - ===.com.error.log
var/www/www-root/data/www/===.com/index.php(122): CWebItem->doModel(), referer: https://===.com/item/new
It returns error 500 or 502 immediately after the announcement is published.
What kind of mistake?

@accept
Enable error logging and check error logs what is there.

After error analysis, solved all the problems by installing PHP CGI 7.4.32
Everything worked clearly except for downloading plugins, it doesn't work through the admin panel, I download it through the server unpacking the archive and only then activate it in the panel.
But this does not interfere with the work.

@AccepT
Coool