all slots flash
Flash with Fastboot | Android Open Source Project
If you flash a new stock firmware, the modem, for example, or one of the other firmwares may be incompatible with an old bootloader (and that's why you need to flash all this stuff along with bootloader on both slots).
How to Unbrick OnePlus 9/Pro/9R/9RT via Fastboot Commands - DroidWin
Hmm, after those steps like fastboot flash abl_b abl.img fastboot flash bluetooth_b bluetooth.img ..... fastboot flash boot_b boot.img fastboot flash init_boot_b init_boot.img.
fastboot commands · GitHub
This in turn will allow you to flash a plentitude of mods and binaries ... STEP 2: Flash Partition Files on Both Slots in FastbootD Mode.