disk space visualizer
https://lifehacker.com/the-best-disk-space-analyzer-for-windows-5915921
The Best Disk Space Analyzer for Windows
Those are tools that show big files/folders as big rectangles, and small files/folders as small rectangles. Really useful when you try to find what to delete.
Surprisingly useful for when you try to open some source code for the first time too, you can instantly see which files matter the most.
A small hint for you. To make those work faster, merge folders with millions of small files into an archive, maybe even an uncompressed zip if it's music / images. Some programs can read files from zips anyway.

This is a long list, but I mostly used 2 of those.
WinDirStat
https://windirstat.net/
and
SpaceSniffer
http://www.uderzo.it/main_products/space_sniffer/
They are nearly the same, but I like how windirstat looks a bit more.