BaseSystem.dmg.x86.trustcache.j###ap.im4m

So I've been trying to deal with some often suspicious files that keep recreating themselves after deleting them to the point I decided to do a complete drive wipe and clean install. However the only other mac I have is old and capped at High Sierra and I need to install Monterey.

So I created the Bootable USB on the same Macbook Pro 2015 that is giving me the issues.

Upon installing, on an external drive 1st to boot and run macos off an external drive, I unmounted the built in to avoid any phishy stuff. However after 30mins of installing on external, it failed saying;

"index.sproduct couldn't be opened"

After it goes back to the Diagnostics select window, and I return to Disk Utility,

A new Disk (not there before) appears called:

Apple UDIF read-only Media

with a volume named: "Shared Support"

What is this disk and volume?

Why does it appear as read only?