[OnePlus 6T  Questions & Answers] Portlandish:
fastboot getvar current-slot

will return with either A / B AS active slot

fastboot --set_active=(Whatever slot your not on)

YOu just need to go to settings, check sys ver. should say OS VERSION somewhere, reboot back to fastboot mode and type above commands. You can also

fast boot boot twrp.img

with this updated official (rename to twrp.img) : https://eu.dl.twrp.me/fajita/twrp-3.3.1-0-fajita.img

permanent twrp installer zip https://eu.dl.twrp.me/fajita/twrp-installer-3.3.1-0-fajita.zip

it will boot then you can copy this zip file to internal and have twrp installed via zip on both slots, you will need to reinstall magisk after on both slots, just make sure to boot into system after you flash magisk the first time before changing slots and then flashing magisk again to inactive slot by fastboot --set_active=(desired slot) and booting twrp img and flash magisk that way..IF U INSTALL THE INSTALLER ZIP(PERM TWRP) ONLY CHANGE SLOTS IN TWRP BY CHOOSING SLOT AND THEN YOU MUST REBOOT BACK TO RECOVERY(OR SYSTEM IF YOU STILL NEED TO VERIFY OOS VER) UNLESS YOU ARE PERFORMING A CLEAN FLASH OF A NEW ROM FROM SAME OOS BASE ON BOTH SLOTS. Then you would have found under settings if you are indeed on same oos on both slots. SO BOTH SLOT A AND B HAVE desired base, you would then boot the img via fastboot boot twrp.img (RENAME TWRP-3.3.0.1-fajita.img to twrp.img) then afterbooting mount all partitions and you would wipe dalvik/cache /system and /data only, (i like to wipe internal sometimes aswell when doing clean flash to make sure device is gutless) then you can remount /system and mount usb storage and copy rom zip to internal and then flash, it will complete and you want to reboot system, it will say no os installed/areyou sure. just say yes and reboot, will boot into custom rom. then you must reboot to bootloader after booting rom once, then fastboot boot twrp.img choose opposite slot (this is the only time i choose to make another slot active while not actually doing a reboot back to recovery because i dont flash the twrp perm-installer untill after i successfully get custom on both slits to avoid recovery issues /i have had many issues before with twrp regarding slot changing and how it flashed to a/b systems with custom roms built in recoverys. so i always do it this way) mount all partitions if not already, again, wipe /system /data /dalvik. you will need to go back and remount all partitions in twrp and then recopy the rom to internal from pc with mount usb storage and then flash rom reboot. then fastboot boot twrp, and install the installer zip to retain twrp forever. if you say ever need to update rom just dirty flash on the slot your on/ reboot system after/ go back to recovery(fastboot boot twrp.img from pc because you may lose twrp-perm after flashing rom depending on rom) choose other slot via twrp in reboot section, then choose reboot system. (recovery--(only works if twrp-perm is installed, or else if you dont perfer having twrp permanently just choose reboot system, then reboot to fastboot after, and reboot twrp.img) then dirty flash on the slot we have just changes to and then rom is updated on both slots, then you may reflash twrp perm(installer).zip and then you need to flash magisk and reboot and then reboot back to recovery and change slots reboot back to recovery and flash magisk once more... YOU MUST FLASH MAGISK AFTER TWRP EVERY TIME THEN REBOOT BEFORE GOING FURTHER INTO STUFF AS ALWAYS OR YOU WILL NOT BOOT . if you want to update to open beta base if still on stock .13 on both slots use above steps instead of /custom rom/ you can be flashing the /beta rom/full ota zips/ on both slots then you will have new base of beta(whatever beta you choose) then you can flsh custom rom on top of that. if you flashed a custom rom over the .13 and wish to update to beta base instead of .13 you will need to reformst the device and lose all data unless you perform a back-up prior