ROS have many distributions, which each one of them runs separately from each other.

Currently the recommended ROS distribution is ROS Melodic Morenia, which is part of ROS1. In addition there is ROS2 and its [releases|https://index.ros.org/doc/ros2/Releases]. Once again each one of them can work separately from each other.

This feature is usefully in case of a new ROS releases. Then it is possible to test if all functionality is still working on the new ROS release before changing a distribution.

If this complexity is not sufficient, ROS make it even more complex. For each ROS distribution, a different set of environment variables need to be set before starting to work with ROS. Those parameters need to be changed in case of switching to a different ROS distribution.

What is already exists is ros-overlay for Gentoo, which supply a solution to this problem, by installing  each ROS distribution at /opt/ros/ (e.g. /opt/ros/melodic)

I would be more then happy to assist in the implementation of the solution. Unfortunately I do not know what is the Funtoo way of solving this package management.