To enter recovery mode in your iPhone it is mandatory to connect it to a computer.
Here there are two ways to enter recovery mode in your iPhone. It varies from the model, that you are using.
1st way
If your iPhone model is iPhone6 or earlier
Connect it to the computer. Then press the Home button and the Side button together. Continually press till the Apple logo is on the screen. Now you are in recovery mode.
If your iPhone model is iPhone7 or 7 Plus
Connect it to the computer. Then Press the Volume button and Side button at the same time. Continually press till the iTunes icon appears. Now you are in recovery mode.
If your iPhone model is iPhone X or 11/12/13
Connect it to the computer. Then quick-press the Volume Up button. Repeat the same with the Volume Down button. Press and hold the Side button till the iTunes logo appears on the screen. Now you are in recovery mode.
2nd way
It is an easy way to put your iPhone into recovery mode. Before that, you have to download the free iPhone Recovery Mode Tool into your computer. Launch the iPhone Recovery Mode Too on your computer and connect your iPhone. After detecting the iPhone device, just click Enter Recovery Mode section. Now you are in recovery mode.
Why do you want the Recovery Mode on your iPhone device?
When your iPhone is not functioning properly, The Recovery Mode is very important. The iPhone shows a software error or wants to upgrade to its latest version of iOS.
If you are having the same error, then you can use alternative apps like 3u Tools.