The About This Mac> Storage section is almost always completely wrong.
Sometime this can be rectified by rebuilding the Spotlight index.
How to rebuild the Spotlight index on your Mac – Apple Support
Open System Preferences> Spotlight Privacy move the icon of your Macintosh HD
on the Desktop to the Privacy panel.
Quit System Preferences.
Open System Preferences> Spotlight> Privacy, now highlight the Macintosh HD icon
and press the minus button.
Spotlight should start to index, to check click on Spotlight in the menubar and type in a word,
if Spotlight is re indexing you should see a progress bar.
This free app, https://www.omnigroup.com/more will give you a text based overview of
the contents of your mac. This will tell you where the largest amounts of space are taken up.
From that you can see which items on your mac you can move to an external drive to free up space.
When you are sure they have copied over successfully then you can delete them from your mac.
Items can be deleted through the app but be careful do not deleteiteyms from your System or Library folders
as this will compromise the running of your mac. Only delete items you understand from within your Users folder.