What is taking up space on my Mac? How to see everything

What is taking up space on my Mac?

The short answer

System Settings shows you categories, and Finder shows folders, but neither shows the whole picture. The fastest complete answer is a map of the disk, hidden folders included. Short of that, the three steps below get you most of the way.

See it without any app

System Settings → General → Storage. The bar shows categories. Click ⓘ next to one to see its biggest items. System Data has no ⓘ, which is why it's a mystery. What's in it →
Finder, with sizes turned on. Open your home folder, switch to list view, then View → Show View Options → Calculate all sizes. Sort by Size.
Show hidden folders. Press ⇧⌘. in Finder. Folders like Library, .docker and .gradle appear. This lists the biggest, and only reads:
```
du -sh ~/* ~/.[!.]* 2>/dev/null | sort -h | tail -15
```

Where the space usually is

System Data: caches, browser and chat files, developer tools
iPhone and iPad backups, often 20 to 60 GB
Photos and Messages attachments
Leftovers of apps you already deleted