Answer by Jahus for Extending the root with another partition (website's server, on production)

Lost space was found by checking fdisk -l. The SSD was on RAID-0.

About moving the website content to the SSD, the solution turned out to be the symbolic link. So here is how I changed place for the website content without downtime and without moving or editing partitions:

1. Copy website's content to a folder in the big partition
   mkdir /home/SiteName/ cp -vrp /var/www/html/* /home/SiteName

2. Move website's folder to a new folder (just in case you need to set back the old folder)
   mv /var/www/html /var/www/html2

3. Create a symbolic link from to the new folder to replace the old folder
   ln -sv /home/SiteName/ /var/www/html

4. Check if symbolic link has been correctly created
   ls -l /var/www/html

5. Delete old files
   rm /var/www/html2 -r

There has been a Wordpress plugin that stopped working after this (Classic Editor) but it got fixed after rebooting the server.

Thanks to members who gave me clues.