The OnePlus 6T uses the newer AB partition scheme first introduced on the Pixel 1. As such there is no recovery partition. Instead, the recovery is part of the boot image. You will temporarily boot TWRP to perform and later perform a more permanent TWRP installation.
First: if you use anything else than a PIN code to unlock your phone, change it now. Last TWRP image for Android 10 asks for a data decryption password. It will be easier if it’s a PIN code. Go to Settings > System > Security and change your screen locking choice by a PIN code.
adb reboot bootloader
You can also achieve this via a key combination: Enter-fastboot-recovery.
fastboot boot twrp-img-name.img
Wait for TWRP to start. Decrypt your data thanks to your PIN code. Swipe to allow system modifications.
Keep in mind that the first boot after wiping the phone may take longer than usual, as the phone will need to rebuild the dalvik cache and initialize other first boot and might reboot a few times.