How do I backup all Photos on my iPhone?

Do you have a desktop Mac? If so it's very easy.

If not, you can back up your entire iPhone to iCloud by purchasing enough iCloud space to support your full library and whatever else you have on your iPhone (you get a free 5 GB of space to start with). Here's how;

To back up your iPhone to iCloud, go to Settings, tap on your name, then select iCloud. Turn on iCloud Backup and tap "Back Up Now" while connected to Wi-Fi.

Be sure you have a strong Wi-Fi connection when you backup.

There are several methods

• First, what I do with my Mac: I use iCloud, so my pictures are automatically copied to iCloud Photos. That's not a backup, since mistakes are copied, too. But those pictures at iCloud are copied to my Mac. That isn't a backup, either, because it copies mistakes. I use Time Machine (since that app is included with Macs) to backup my Mac, every day, so I can go back and see my Photos Library yesterday, last week, last month-- before I made the mistakes. A big advantage with iCloud is that you don't just backup pictures-- you backup the Library which has both the unmodified original files as well as the editing and commenting. There are other backup apps for computers of different ethnicities, as well.
• If you don't use iCloud at all, then you can connect an iPhone with a USB/lightning cable to a computer and copy pictures over directly. The simplest method doesn't get you unmodified originals, but generally smaller versions of pictures. There are apps you can get (probably by subscription) that will do better.
• You can also plug a hard drive or a flash drive directly into an iPhone. This is easiest with the newer phones that have a USB-c port. The Files app will show the external Drive. (In lightning phones, I've had trouble with that part.) Then you transfer pictures to the Drive by using Share>Save to Files.
• On a Mac, you can use Finder to back up all the data on your phone.
• For Windows, try reading this:

What Apple says:

Transfer photos and videos from your iPhone or iPad to your Mac or PC - Apple Support

What Microsoft says :

https://support.microsoft.com/en-us/windows/import-photos-and-videos-from-an-iphone-to-pc-2e4e4db4-4c3d-041c-b88f-3ee4358dd95e

There's also this:

Back up iPhone - Apple Support