I thinks this is all normal. With the Pixel 3, Google changed from using EXT4 to F2FS, which is more secure:
https://www.anandtech.com/show/13474/the-google-pixel-3-review/2
I suspect rootfs is simply what they called the system storage filesystem prior to this change.
EDIT: Actually, I'm trying to figure out if rootfs appears only if the phone is rooted. You said you didn't, but where did you get this phone from? Is it possible the prior owner may have rooted it? Have you tried running a root check app?
Seems like it's needed for postmarket OS installation:
https://wiki.postmarketos.org/index...google-pixel)&mobileaction=toggle_view_mobile