The phone does not appear in File Explorer. What am I missing?

On Android it is nearly always the USB mode. Pull down the notification panel on the phone, tap the USB notification and choose File transfer. On iPhone you have to answer Trust and type the phone's passcode, otherwise the phone stays locked to the computer.

Where are the photos inside the phone's folders?

In the DCIM folder. On iPhone they sit in subfolders named things like 100APPLE. On Android, camera photos are in DCIM and then Camera, while pictures you were sent in messages are often in the Pictures folder.

Can I delete the photos from the phone once the copy finishes?

Wait until you have opened a dozen of them on the computer and seen that they display properly. If a transfer is interrupted, files can look like they are there but be empty, and that only shows when you try to open them.

Can I do it without a cable?

Yes. If Google Photos or iCloud Photos is switched on, your pictures are already in the cloud. Go to photos.google.com or icloud.com on the computer, select what you want and download it. On a Mac you can also use AirDrop from the phone.

How to move photos from your phone to your computer

Copying your photos to a computer is the surest way not to lose them. With a cable it takes about ten minutes for a few thousand photos. The rule is simple: copy first, check second, delete last.

Find the cable that came with the phone. Some cheap cables can only charge and not transfer files, so if the computer does not react at all, try a different cable before you look for anything else.

Try this first

Connect the phone to the computer with the cable and unlock the phone's screen.
On iPhone, answer Trust to the question that appears and type the phone's passcode. Without that the computer sees nothing.
On Android, pull down the notification panel, tap the USB notification and choose File transfer (sometimes called MTP). This is the step almost everyone misses.
On a Windows computer, open File Explorer. The phone appears in the left hand column under its name, such as Apple iPhone or Galaxy.
Open Internal Storage > DCIM. On Android, carry on into the Camera folder. Your photos are in there.
Select what you want, right click and choose Copy, then paste it into a folder on the computer. It helps to make a folder named Photos and the year.
Let the copy finish without pulling out the cable. The box on screen shows how many files are left.

On a Mac

Connect the iPhone with a cable and answer Trust.
Open the Photos app, select the phone in the sidebar and click Import All New Items.
Or select the photos on the phone, tap the share icon and choose the computer's name under AirDrop. The computer needs Bluetooth and Wi-Fi switched on.
An Android phone connects to a Mac using the Android File Transfer app, which you download free from Google's website.

If that did not help

Try a different cable and a different USB socket on the computer, preferably one on the computer itself rather than on a monitor or keyboard.
Use Photos in Windows instead of File Explorer. Open the Photos app, choose Import and then From a connected device.
Fetch the photos from the cloud instead. Go to photos.google.com or icloud.com in the browser on your computer, sign in, select the photos and choose Download. Type the password yourself and never share it with anyone.
Open a dozen of the copied photos on the computer and check they display.
Copy the folder onward to a USB stick or an external hard drive. Computers break too, and two copies in two places is the rule that holds.
If Windows will not display HEIC photos from an iPhone, set the phone to Settings > Camera > Formats > Most Compatible. New photos are then saved as ordinary jpeg files that every computer can open.

Delete nothing from the phone until you have seen the photos open on the computer. If you have many years of photos and want them organised, or the phone will not show up on the computer despite the right USB mode, write to us and we will do it together.