What’s booting by USB?
The process of booting by USB is similar to booting by Windows. The difference is that booting by Windows is based on the system of the local disk while booting by USB uses the system on the USB to boot your computer.
After you create the USB rescue disk, you need to get into BIOS to set the USB as the prior boot device to boot. Then you can get into the USB rescue disk and use its functions.