Pictatasu In at least 99% of cases you can obtain the driver for fastboot simply by connecting the device while it's in the bootloader interface, going to Windows Updates, clicking "Check for updates", waiting for the update check to complete, then going to "Optional updates" within Windows Updates to install the driver. The driver name will include "Android bootloader interface", and perhaps start with "Lemobile".
On Windows 11, the procedure to access "Optional updates" is to first select "Advanced options" within Windows Updates.
The behaviour described in the OP has since changed: with a fully up-to-date Windows install you will now be able to unlock the bootloader and start the flashing without the driver – although without the driver it won't finish the flashing, it'll just get stuck at the fastbootd screen on the device, while the WebUSB installer says "Restarting device". You can simply install the Android bootloader interface driver from Windows Updates. If there is no driver, check for updates again.