@sAmboy13
Once you are in fastboot mode at the screen that reads "START" use your volume rockers to navigate up or down and go to screen that reads "POWER" and use your power button to then select that and your device should turn off and you can then connect your device how the directions say and your PC should see your phone and in device manager your OnePlus 6t will be labeled Qualcomm USB driver or something like that.
User who commented after my post here is correct. And any time your using crappy windows best believe the Qualcomm USB drivers are not going to install smoothly. I had to uninstall mine and reinstall to get them to work.
I believe firehose has something to do with the connection between your device and the computer, like a connection. I know on Motorola devices firehose means your PC is attempting to communicate with your device...
(Edit)