Perhaps you are referring to this article: https://support.apple.com/en-us/HT207022#working
HEIC and HEVC are great for saving storage in iCloud and on iOS devices, like iPhones or iPads. But on the Mac they will require extra storage.
Even if you can avoid importing of an additional JPEG version from your iPhone or iPad by switching from My Photo Stream to iCloud Photo Library, the Mac will store additional versions as previews for the Media Browser in your user library.
For each and every HEIC image or HEVC video, there I´will be an additional JPEG or MOV stored in the folder /Users/<username>/Library/Containers/com.apple.MediaLibraryService/Data/Library /Caches/com.apple.iLifeMediaBrowser.ILPhotosTranscodeCache/
Even if your Photos Library is on an external drive, this folder will eat up storage on your system drive.
But I import all new photos and videos to my library in HEIC and HEVC format nevertheless.