I have the latest version of nextcloud on ubuntu dekstop and i need to see files that users store on nextcloud what path in the file explorer to see the files
Are you referring to the client, or have you installed nc server on a ubuntu desktop? 
Installed server on Ubuntu desktop
By default the data folder is kept in the installation directory, so with Apache it would be /var/www/html/data or /var/www/html/nextcloud/data
In there will be a list of users, and their data will be in the files subfolder of each user.