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. 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
Delete items, and empty the trash.
This is the only way to recover storage used by Gmail or Drive.
There are many complaints that it may take very long before the storage is recovered after deleting photos. Also note that, when you empty the trash, there is no way back. It is strongly recommended to have a real backup of your photos before you start deleting.
Use "Recover storage" on https://photos.google.com/settings. That will convert ALL (it's all or nothing) photos/videos to "Storage saver" quality, using substantially less storage. Save the ones you want to keep in original size somewhere else before you use this option.
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.
Delete unsupported videos on https://photos.google.com/unsupportedvideos (or via "Settings")
Check on https://get.google.com/albumarchive whether there are no photos/videos that are not shown in Google Photos, for example from Hangouts, which may use storage.
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 Gmail possible). See "Contact us" at the bottom here: https://support.google.com/drive/?hl=en#topic=14940. Some users got temporary extra storage.