Most of what fills a Mac is designed to grow back. Caches refill as you use your apps, chat apps download attachments again, iCloud downloads files you open, and developer tools rebuild their folders. A one-time cleanup works for a few weeks. Then you're back where you started.
What refills fastest
Browser and app caches. They start growing again the next time you open the app.
Chat app files. Every image in a busy Slack or WhatsApp group is saved on your Mac, and Messages keeps every attachment forever unless you tell it not to.
iCloud Drive and Photos. Every file you open is downloaded and kept, unless Optimize Mac Storage is on.
Developer folders. Xcode, Gradle, npm and Docker can add gigabytes a week.
Slow it down, without any app
In System Settings → General → Storage, turn on Empty Trash Automatically.
Turn on Optimize Mac Storage for iCloud Drive and for Photos. How →
Write down your free space once a week. If it drops by more than 5 GB, something is growing fast.