You can try using du -h -d 1 / to locate the largest directory under /. Once you’ve located the largest directory, replace / with that directory. Repeat that until you find the culprit (if there is a single large directory).
EDIT (2024-07-22T19:34Z): As suggested by @DarkThoughts@fedia.io, you can also use a program like Filelight, which provides a visual and more comprehensive breakdown of the sizes of directories.
It’s “Steam” inside .local eat up 6GB even though I have not open it yet and tmp files (almost 5GB) that is not clear itself after installing the OS
Or you could use baobab to do the same thing if you want an answer within 10 minutes.
Whoops! You are correct — I have updated the original comment. I’m not sure why I wrote df instead of du. This is a good example of why one should always be wary of blindly copying commands 😜 It begins to teeter on being potentially disastrous if I had instead wrote dd.