If you use Google Photos and the trashcan icon, that’s one kind of delete. If you choose the menu and “Delete from Device”, that’s another kind of delete. And, if you use a photo gallery app other than Google Photos to delete, that is yet another kind of delete. It is so important to know what you are deleting when you press the button! Yes, it can be confusing, make sure you have an extra backup first. This is a common question with Google Photos. For lots more detail, see the official help page on Google Photos for deleting.
First, for some background
When you take a photo or video on your phone, it is stored on your phone. If you have enabled the Google Photos Backup feature, a copy of the photo is uploaded to the cloud where it is stored on a remote Google server. This allows you to access your photos from any device that is connected to the internet – just go to the website Photos.Google.com. With your Google account, you get 15GB of free cloud storage, then you need to pay a nominal amount for more. Some people would rather delete photos from their Google account (in the cloud) than pay for more storage. But they do not want to delete the device copy. Other people are running out of storage on their phone and want to delete that copy without deleting cloud copy.