[OnePlus 7 Pro ROMs, Kernels, Recoveries, & Other D] mauronofrio:
So i will try to explain all the procedures and the problems we have with oneplus 7 pro (and i think also with the others versions)

NORMAL GUIDE

1. Install fastboot tools from google's sdk on your PC

2. Enter fastboot mode

3. Unlock your bootloader

4. Run "fastboot boot recoveryimgfilename.img" in command line

5. Copy the recoveryimgfilename.img in the phone memory

6. Go in advanced menu

7. Click on "Install Recovery Ramdisk"

[IMG="ImageGuide"]https://i.postimg.cc/vBxYFq2V/Screenshot-2019-04-04-17-25-43.png[/IMG]

8. Choose the recoveryimgfilename.img

9. Swipe to install.

Like you can see I never mentioned this method, because "fastboot boot " command DOESN'T WORK PROPERLY

NORMAL GUIDE 2

1. Install fastboot tools from google's sdk on your PC

2. Enter fastboot mode

3. Unlock your bootloader

4. Run "fastboot boot recoveryimgfilename.img" in command line (if you want to have it permanently you should flash the installer)

5. If you will flash the installer and you are stock you need also to flash magisk or your phone will not boot again

6. Enjoy it:)

This is a second Normal Guide i mentioned for 1 day when the "fastboot boot " command had been restored. Only the day after we realized that it still had problems and I decided to remove this guide. DOESN'T WORK PROPERLY

OLD GUIDE

1. Install fastboot tools from google's sdk on your PC

2. Enter fastboot mode

3. Unlock your bootloader

4. Now you should check which slot are you using with this command "fastboot getvar current-slot"

5. Now you should change the slot in use with this command "fastboot --set-active=*" you should replace with the slot is not in use, for example if you are using slot a the command will be "fastboot --set-active=b"

6. Now you should flash the twrp in the boot partion using this command "fastboot flash boot twrp.img"

7. Now reboot the device with in recovery mode scrolling the menu with the volume button and selecting "Recovery Mode" with the power button

8. First thing go in reboot menu and restore the old active slot

9. Now you can flash the installer (Actually you can do it using sideload, start sideload server from twrp advanced menu and from pc use this command "adb sideload twrpinstaller.zip")

If you are already rooted, you can skip all and directly flash the installer like a magisk module. Always remember that after you flashed the installer you will be unrooted.

I found this guide in the Essential Phone threads, because also on the Essential Phone "fastboot boot " command is not working.

After some days i understood that changing the slot to use the inactive slot on the oneplus 7 pro it didn't work well, because after the reboot the old active slot was restored, so you flashed the twrp only on the inactive slot and and you always booted the active slot. So this method DOESN'T WORK PROPERLY.

ACTUAL GUIDE:

1. Follow this guide to root your device: https://xdaforums.com/oneplus-7-pro/how-to/guide-root-oneplus-7-pro-patched-boot-t3931205

2. Flash the TWRP installer like a magisk module

3. Flashing the TWRP installer you will lose the root, so you will need to flash magisk again from magisk manager before reboot the device or you can reboot the device and flash magisk in twrp

Thanks to [MENTION=4544860]osm0sis[/MENTION] we can flash the twrp installer in magisk manager so we can bypass the "fastboot boot " command just rooting the phone. THIS IS THE ONLY SAFE METHOD.

Now let's explain something about A/B devices.

A/B devices is a wrong name and creates confusion, this name is only used to identify them.

A/B devices = devices that support seamless update

Documentation: https://source.android.com/devices/tech/ota/ab

Fast explanation:

These devices have 2 slots for every partition can be updated.

So for example system has 2 slots, boot has 2 slots, vendor has 2 slots, but userdata and persist for example have only 1 slot, these because they can't be updated ( persist is a static partition and userdata contains apps and files).

Why this?

Easy, for the updates. When you receive an update it will be flashed on the inactive slot without entering in the stock recovery, because is not more needed. After a reboot the inactive slot will become the new active slot.

Actually seems the recovery stock is not more needed, but this is not true, it is needed for wipes.

But now the recovery stock don't need anymore of a dedicated partition (because it is used only for wipes) and it is included in the boot partition.

The boot partition is divided in 2 parts, ramdisk ( where there is the recovery) and kernel.

So how to to flash a recovery without overwrite a kernel?

Just taking the boot partition image, dividing ramdisk and kernel and creating a new boot image with the twrp ramdisk and the kernel was already in the boot partition and then flashing it. So you will have your kernel with the twrp. All this process is made from the twrp installer or from "install ramdisk" option in twrp.

Now the magisk trick. When you flash magisk, it will modify the boot ramdisk adding something for the root and disabling some controls, so better, when you flash the twrp installer, your ramdisk will be overwritten, when you flash magisk your ramdisk will be only edited. We come to the point. Flashing the twrp installer you will lose all the magisk changes and you will need to flash it again.

Now how you can flash magisk again:

if you are using magisk manager to flash a twrp installer, after the flash you need only to click on "install" and then on "direct install"

If you are only updating the twrp through a twrp installer in twrp,after it you can flash magisk.zip.

If you are updating the stock rom using phone settings, after flashed the twrp installer (if you want to keep also the twrp) you can click on "install" and then select "inactive slot" so magisk will be flashed on the slot where the update was flashed and it will be the slot you will use like default after a reboot.

I think that's all.

I think also that all this can be found easily by searching on google or even here on xda.

I hope I have resolved your doubts.

I just add some information: Official TWRP is coming, stay tuned.

P.s. I forget to say a thing, with seamless update, cache partition doesn't exist anymore, so you will not find it on twrp. ( cache is stored in /data partition so when you wipe data, also cache will be wiped)