How can I find purgable storage and purge it?

Purgeable storage are items that the OS will delete when space is needed (local snapshots) or items that can be restored from another location, like iCloud Drive.

So, if you want to purge the purgeable items, you can go through and find files you have stored both locally and on iCloud Drive and remove the local copies. Depending on how they are stored, you may end up deleting both copies if you don't know what you are doing.

Purgeable storage is not items you don't want or need.

Here is how to remove Time Machine local snapshots: https://eshop.macsales.com/blog/77372-how-to-delete-time-machine-local-snapshots/