Hello,
I was wondering, is there a way to create user roles or something that allows to have a user that can create and manage users and groups but can’t have access to the “Applications” functions ?, I mean, this user can’t install and uninstall apps and plugins.
Nextcloud Version: 13.0.1
You can set a User as a Group Admin.
Group admins are granted administrative privileges on specific groups, and can add and remove users from their groups.
https://docs.nextcloud.com/server/13/admin_manual/configuration_user/user_configuration.html
Oh yes, so there is not a way to do exactly what I want. Someone asks me for a superuser with no access to Applications management 
It is exactly that what you asked for. Unfortunately without the function to create and manage groups.