1. On installation there is already an error message in all menu items except dashboard:

Warning: Declaration of JDRoute::_($url, $xhtml = false, $ssl = NULL) should be compatible with Joomla\CMS\Router\Route::_($url, $xhtml = true, $tls = self::TLS_IGNORE, $absolute = false) in /home1/site/public_html/plugins/system/jd/jd/jd/route.php on line 11

What does it mean and how is it fixed?

2. I anyways created a downloads product, but the next problem when trying and failing to upload the file is this Error:

This file type is not allowed

How do I allow my file types? Didn't find any instructions anywhere. Tried adding it to joomla allowed media files but it didn't work (it's a trading file for MT4 ext e.g. filename.ex4)

3. Your tutorial Documentation seems to be for some other version, as your images doesn't match v 1.1.2 free.

Also at least these plugins don't exist in my site like:
Joom Downloads Fields Plugin
Joom Downloads for Articles

Release Details Configure Download access does not exist.

4. This plugin is missing in your menu, and also in your forum dropdown list.

I hope to get some help and that this wasn't a complete waste of time.

I run latest Joomla 3.9.12. PHP 7.1.14 and MySQL 5.6.41-84.1