Disk Usage Analyzer (also known as baobab ) scans folders, devices or remote locations and and reports on the disk space consumed by each element. It provides both a tree-like and a graphical representation.
I use this all time, to find where all my files are (see the screenshots at Migrating from Ubuntu 16.04 to Fedora 28). It is a useful app to get acquainted with a new filesystem, I wish it were installed on all PCs.
Code and tracker at Projects · GNOME / baobab · GitLab. A great piece of the GNOME project.