Where did my finder arrow near my trashcan go?

I'm not sure if this is what you are describing but if you want an icon in the Dock that will open ~/Downloads navigate to your User Account's Downloads folder and drag its window icon to the Dock. It will not resemble that "arrow" icon though — I think that may have been a shortcut (alias) to Downloads. It will work either way.

Refer to Create and remove aliases on Mac - Apple Support

It is unclear just what you want here.

You can drag any folder you want to that area next to the trash can and after the vertical line.

If you hover your mouse pointer on the icon that looks like a stack of photos, it will identify the folder (maybe it's Downloads, I don't know). You can click on that icon to expand. And if you want other folders in there, just drag them.

One thing you may consider and based upon your user By-line " MacBook Pro 13″, macOS 14.2 "

This machine needs either an UpDate to current Sonoma 14.7.4

Or, if it qualifies macOS Sequoia compatible Computers which is currently version 15.3.1

In either case, always make a Time Machine Backup  before doing any upDates / upGrades