How can I close all of my open apps at once?

You cannot. There is no way to close all apps at once on an iPhone.

You generally don't need to close apps unless they are having issues, are frozen or crashed.

You can offer feedback to Apple if you would like to see this feature by clicking here ➜ Feedback - iPhone - Apple

Ckokka wrote:

How can I close all of my open apps at once?

Closing apps can have a deleterious effect on both battery life and performance. As Phila0124 noted, the only tie you really need to do that is is you are having issues with a particular app.

You can’t and I’d really like to see evidence that shows closing all apps causes harm to the device. I recently started closing all apps on my iPhone 12 Pro Max and I’ve actually been having a better experience in terms of speed and battery life.

hey_thisisandrew wrote:

You can’t and I’d really like to see evidence that shows closing all apps causes harm to the device. I recently started closing all apps on my iPhone 12 Pro Max and I’ve actually been having a better experience in terms of speed and battery life.

It doesn't harm anything. However, it can have a negative effect on your battery life. But, in the end, it's your time to waste.