How to uninstall apps on a Mac, leftovers included

Quit the app, then drag it from Applications to the Trash, or use its own uninstaller if it came with one. That removes the app but not what it left in your Library: preferences, containers, caches and login items. Find those by the app's bundle ID, check them, then move them to the Trash too.

The three ways to remove the app itself

Finder. Quit the app, open Applications, and drag it to the Trash. This works for almost every app.
Storage settings. System Settings → General → Storage → Applications lists your apps by size, with a Delete button. It is the quickest way to see which apps are worth removing.
The app's own uninstaller. Apps that install drivers, system extensions or background services (Adobe's apps, VPNs, security tools, audio drivers) usually ship one. Use it: it knows where its own pieces are, and some of them need an administrator password to remove.

Empty the Trash afterwards. Until you do, nothing is freed.

What dragging to the Trash leaves behind

The app bundle is only part of an app. While you used it, it wrote to your Library, and those files stay after the app is gone:

~/Library/Application Support/ — the app's data. Often the biggest piece.
~/Library/Containers/ and ~/Library/Group Containers/ — where sandboxed apps keep everything.
~/Library/Caches/ — regenerable, but it does not regenerate for an app that is gone.
~/Library/Preferences/ — small .plist settings files.
~/Library/LaunchAgents/ and /Library/LaunchDaemons/ — helpers that start on their own, sometimes still running after the app is deleted.

Finding the leftovers by hand

Most of those files are named after the app's bundle ID, not its name. Get the ID before you delete the app:

mdls -name kMDItemCFBundleIdentifier /Applications/AppName.app

Then look for anything carrying it:

find ~/Library /Library -iname "*com.example.appname*" 2>/dev/null

Check each result before you delete it, and move it to the Trash rather than deleting it outright, so a mistake can be put back. Some apps file themselves under their plain name instead, so search ~/Library/Application Support for that too.

What to leave alone

Apple's own apps. Most are part of the sealed system volume and cannot be removed; the ones that can are reinstalled from the App Store.
Shared folders. A vendor's folder in Application Support (Google, Microsoft, Adobe) may hold data for apps you are keeping.
Anything you cannot identify. A helper with a name you do not recognise may belong to an app you still use.
Your own data. Some apps keep your only copy of your work in Application Support. Export or back it up first.

Before you uninstall to save space

Look at the size first. Storage settings shows the app; the leftovers are often as large again, and sometimes much larger, and they are counted under System Data rather than with the app. If you are clearing space rather than an app you dislike, start with the apps you have not opened in months: they are the safe ones to lose. For the other places worth checking, see how to free up space, or work through the clean-up checklist.

Where the app comes in

RefreshYourMac's Uninstall an App does the search above for you. It looks for the app's bundle ID and those of its helpers, login items and XPC services across both Library folders, shows every piece with its size, and removes nothing until you press the button. Everything in your own Library goes to the Trash, so an uninstall can be undone until you empty it. Anything it will not touch is listed under Left alone, with the reason, and Apple's own apps are refused outright. Its Unused Applications category lists the apps you have not opened in a while, by their real last-used date.