Scenario in short:
I’m trying to install LineageOS onto a spare phone that I have. Hoping to get a daily driver out of it. The installation process requires me to install Android11 to install lineage on top of /over that. Which afaik isn’t directly available outside of some websites that I’ve never heard of.
For safety’s sake: would the install of lineage wipe the install of Android11? Or could there still remain backdoors?


Normally LineageOS replaces the version of Android previously installed on the phone.
I don’t know why the process asks for Android 11 as a starting point. There are a couple of valid reasons why you might need this, depending on the device.
Can you share a link to the process description?
At least in the case of my phone I had to put 11 back on to unlock the bootloader because OnePlus removed the ability to do it in their version of 12. Screw those guys.
AFAIK, custom ROMs ask for specific OS to get specific firmware for some peripherals (modem, WiFi, BT, etc).
E.g. installing Ubuntu Touch on Nord N10 5G requires a downgrade to OxygenOS 10 using the official tool, which (i believe) installs original firmwares.