Google Photos does not create a true backup, especially not for photos on mobile devices:
Deleting from Google Photos also deletes from synced mobile devices.
Deleting from mobile device leaves you with only one copy which is never safe.
Please read this article by Geeks on Tour, Google Photos is NOT a "Backup" of Your Photos
Photos that are on your computer
If your goal is to back up all your photos and files (such as Word, Excel, PDF etc), you can use external hard drives or thumb drives to back up everything on your computer, either manually or using a backup system that makes monthly and daily backups (full and incremental backups). You can use the free version of Macrium Reflect or EaseUS ToDo or any other backup program. Search the Internet for directions how to set up these systems.
Store at least one backup away from home to protect from flooding, fire or other disasters, or consider backing up your files also to another cloud service. I would consider Google Photos only as a "Backup of last resort".
Photos from mobile devices
Google Photos has no real backup feature (save a non-synced copy). There are some work-arounds:
You can copy the photos directly from the device to a computer or external drive, for example by using a USB cable. Search the Internet or read the manual that came with the device for more detailed information.
You can download the photos that are in Google Photos to a computer or external drive. See this link.
You can add the photos to another cloud service.
Copy to Drive (you may have to pay for more storage):
https://www.youtube.com/watch?v=e4x42gsuGBQ (Chris Guld)
https://support.google.com/photos/thread/11728028 (Larry Paul)
Automatically upload to a second cloud service like OneDrive. Office 365 users have plenty of free storage. Or you can use the free storage OneDrive offers as a temporary storage till another backup is made.
See more info about saving to OneDrive for Android or iOS.
An additional backup can be made by sharing your library with a second Google account, and activate the option to save photos to the library of account 2. When you loose/delete photos accidentally on account 1, they will not be deleted from account 2.
The photos will not use additional storage on account 2, even when uploaded as Original, till they are deleted from account 1. When deleted from account 1, they will start using storage according to the rules in force at that time:
Till June 1, 2021 for free if uploaded with setting "High Quality"
After that date the newly added photos will start counting for storage (less when you have setting "Storage saver" selected in https://photos.google.com/settings