When I connect via USB (Android to PC), it automatically starts charging.
According to what I've read, a 'File Transfer' window should open. Mine just says, 'slow charging'.
How do I transfer files?
This cable could be a charging-only cable, which doesn't include the D+ & D- cable streams. If you're unsure of the type of cable, try using a different cable that you're sure works.
USB 2.0 minimum cable requirements were easier to manufacture because of the lower data rate requirements. Ever since USB 3.0, lots of USB 3.0 cables don't meet "spec" and therefore can not work or damage your phone. Make sure you have a brand that you know is good.
From Android 6.0 onwards, file transfer is disabled by default when a cable is connected to the device, mainly due to malicious USB hubs which pretend to be normal charging ports, but instead copy data from your device whilst charging.
Turn on your device and connect it to your computer (using a cable which supports data transfer)
Unlock your device and open the notification labeled "USB charging this device" - you may need to swipe down from the notification bar
Select "Transfer Files" from the menu that appears.
You should be able to access your device on your computer.
I tried a bunch of cables on one USB port but nothing was working. So I tried a different USB port and it all worked again!
Some might say this is the first thing to try.
Make sure your wire supports both charging and data.
If it does, then on the phone go to Settings -> Storage -> Overflow menu (3 dots) -> USB Computer Connection -> Change the mode from Charging Only to MTP or USB Mass Storage.
If neither of these works, then make sure you have drivers installed for your device on your computer.
Generally, whenever you connect the USB, the phone is in USB charging mode by default.
To change to any other mode (File Transfer Mode in your case), go to the notification pane where a notification will tell you that USB is connected and charging. Tap on it and select the MTP or File Transfer Mode.