It worked out pretty well from what I experienced. I am not that handy with the commands from the command prompt because I only mess around with it when i'm rooting a phone and had extra problems because for some reason this phone could not drag and drop files from my PC when connected. My one plus works fine with the same cable but I am assuming there are issues with the pixel 3 driver similar to pixel c. Soo with that being said, I had to use command prompts for sending the files back and forth and at one point i just installed mega on the phone to drop the modified boot image to the computer to flash it on the phone.
I had a couple reasons for this. First is that I did not want to login to this device with my google account. I am a sprint customer and I wanted to activate this device with a t-mobile esim so I can send it to my mom since she is on my plan. Since the device has been registered on my account, and on the network, I can now send it to her and activate it on her line when she gets it. I also wanted to root the phone and have ad blockers, you tube vanced, vpn hotspot, and a couple other beneficial apps on there instead of getting her a phone full of bloatware and limited capabilities. (or else I would have got her an iphone se, lol). That is the reason for magisk, even though the unlocked pixel 3 is nice with very little bloatware on it and the installed tmobile apps that came with the 12 update were very easily removed.
In regards to getting twrp installed, I did brick the device once because i flashed it to the other partition, which is a bad move. I reflashed from the google official image to upgrade to 12. Don't change from A to B in twrp when you are in there, just click boot to system and thats it. There is NO need to switch partitions. I just wanted to make sure this device is 'unattended' not like a tethered jailbreak. It also made me realize to re-root my pixel c, I will have to do something similar with sending that boot image back and forth, but once its done, good riddance. So the big challenge was sending the files back and forth from the device, but if yours loads up in your computer it will be way easier. Mine only shows up if i switch it camera mode, and only then the photo folder, but maybe its a defect from buying a used/refurbished phone. I mean it shows up in adb, but not as a device with folders in my windows explorer for whatever reason
This article was used as reference for pulling the files back and forth:
ADB push and ADB pull are solid file transfer techniques for your Android customization experience, even if there are easier ways to get files between your Android device and a PC.
www.androidauthority.com