You can find info about storage used and files using most storage in different places. It's not clear why there are so many, partially overlapping, but none is complete, and none suggests to use "Recover storage" in https://photos.google.com/settings to reduce storage used by photos/videos uploaded with setting "Original." It's recommended to look at all of them.
https://one.google.com/storage how much storage is used by Drive, Gmail or Photos
https://photos.google.com/quotamanagement shows large files, blurry photos, screenshots
https://one.google.com/storage/management shows deleted (= in trash bin), large and other items like "unsupported videos"using most storage.
https://drive.google.com/drive/quota largest files in Drive
You can delete items in Google Photos (and trash!), but there are many complaints that it may take very long before the storage is recovered. Also note that deleting items uploaded with setting "High quality" before June 1, 2021 will not recover any storage, because these photos do not use any.
WARNING: Deleting from Google Photos may also delete the device copy.
See Delete from Google Photos but not from mobile device
"Recover storage" on https://photos.google.com/settings converts ALL (it's all or nothing) photos/videos uploaded with setting "Original" to the former "High Quality" - now called "Storage saver."
It seems that Google One accounts do not show the "Recover storage" button, but you can still get there by toggling between Original and Storage saver settings.
See example in this album (no longer correct for Google One users)
Save the photos/videos you want to keep in original size somewhere else before you use this option. High-quality videos will suffer from compression.
Important: It seems that photos from many cameras are NOT compressed when uploaded with setting "Storage saver" due to the presence of an MPF flag (CIPA Multi-Picture Format) which stores multiple sizes of a photo into a single JPEG file. In the past, these photos were not compressed, but also not counted toward used storage. Since June 1, 2021, however, they do count for the full uncompressed size. A work-around is to resize/compress the photos or remove the MPF flag before uploading.
See https://support.google.com/photos/thread/124062039/google-photos-compression-doesn-t-work and https://support.google.com/photos/thread/115585724/storage-saver-doesn-t-compress-uploaded-image-to-16mp
The folder "Google Photos" in Google Drive starts using storage when the corresponding photos in Google Photos are deleted. Thus, when you delete from Photos, you may also have to delete the Google Photos folder in https://drive.google.com/drive/my-drive if you have one. Note that this is a legacy folder from the past and is no longer updated.
Delete unsupported videos on https://photos.google.com/unsupportedvideos (or via "Settings")
You can also check on https://get.google.com/albumarchive. There may be photos using storage that are not shown in Google Photos, for example photos from Hangouts.
Google Photos has no live help, but when you pay for storage you can contact the Google One Help.
You can also try Google Drive Help when you are really in trouble (no more use of Gmail possible). See "Contact us" at the bottom here: https://support.google.com/drive/?hl=en#topic=14940. Other users were given some extra temporary storage.