[OnePlus 6T  ROMs, Kernels, Recoveries, & Other Dev] mxz55:
Extracting the firmware updates from the new ROM (OxygenOS) to flash into your LineageOS phone, is easy as well.. it's just more time consuming than taking the files from someone that already extracted them, like my first post was for.

For the sake of completion here is how to do that:

1) Get the updated OxygenOS ROM, it's a zip file like in this case called OnePlus6TOxygen_34.J.62_OTA_0620_all_2111252336_339a2fa8335f21

Make sure to get it from an official source, i dont know where OnePlus shares direct links but i know that their practises regarding it are inconsistent (sometimes they provide, other times they don't). So i advise you to use an useful app in the Play store called Oxygen Updater that fetches it directly from OnePlus servers.

For this app.. if you're on LineageOS, you can click through to download the ROM update file but only after ignoring all warning dialogs saying things like "Unsupported device" and about root + enabling Advanced mode slider in the app's settings. So basically, these steps override 1) that you're on LineageOS (so it thinks it's unsupported) and 2) that it doesn't recognize your current version and thus doesn't offer a direct download of the file, regardless of what it fails to detect about your phone or OS, to your phone storage.. that's what Advanced mode takes care of. So after enabling Advanced mode, download the ROM .zip from the update screen in the app, and it will be placed in your phone's storage root folder

So now that you have the ROM, let's start the firmware extracting process -

Required tool: payload-dumper-go > choose latest version and download the Windows package

1) Extract payload-dumper-go to your folder "FW update"

2) Move the ROM .zip file (downloaded from Oxygen Updater app) from your phone storage to your PC, and place it in that "FW update" folder

3) Open the ROM .zip file with an archiving program, like 7-Zip

4) Extract payload.bin from within the ROM .zip file using 7-Zip, this .bin file should display as shown below:

So after extracting this from the ROM .zip, you open up cmd (command prompt of Windows) and write cd [folder "FW update" location) or even better, copy cmd.exe from System32 to your "FW update" folder. I always use that method for all my maintenance, e.g it's copied to my "adb" folder as well for easy folder targeting

4) Now in cmd, use payload-dumper-go to pull firmwares from payload.bin:

Enter each command separately, after the other completes

payload-dumper-go -p abl payload.bin
payload-dumper-go -p aop payload.bin
payload-dumper-go -p bluetooth payload.bin
payload-dumper-go -p cmnlib payload.bin
payload-dumper-go -p cmnlib64 payload.bin
payload-dumper-go -p devcfg payload.bin
payload-dumper-go -p dsp payload.bin
payload-dumper-go -p fw_4j1ed payload.bin
payload-dumper-go -p fw_4u1ea payload.bin
payload-dumper-go -p hyp payload.bin
payload-dumper-go -p keymaster payload.bin
payload-dumper-go -p LOGO payload.bin
payload-dumper-go -p modem payload.bin
payload-dumper-go -p oem_stanvbk payload.bin
payload-dumper-go -p qupfw payload.bin
payload-dumper-go -p storsec payload.bin
payload-dumper-go -p tz payload.bin
payload-dumper-go -p vendor payload.bin
payload-dumper-go -p xbl_config payload.bin
payload-dumper-go -p xbl payload.bin

Now they are pulled, but will be automatically put in folder names (as subfolders to your "FW update" folder) that look like this: "extracted_20211216_135146" with unique number each. These folders contain just 1 pulled firmware (.img files) individually

Now you have to move all .img files from each of such folders, to your main directory ("FW update"). I prefer to do it like this: Windows Explorer search for *.img like below image

Select all matching results from this search and copy them to "FW update" or put them in a new subfolder "Pulled firmwares"

Now the firmware extracting process is complete. From this point, the flashing process should be picked up, the one from my first post in this topic

Edit 18:32 - i decided to post full fledged guides on both subjects from my posts here, on how to extract the firmwares and also how to flash them:

[GUIDE] Extracting the latest firmwares from ROM update package

[GUIDE] Flashing the latest firmware updates to your OnePlus 6T

So if you have any comments about the instructions, please take them to the guide topics