Sidebar and Toolbar Missing in Finder

Look at the keyboard shortcuts in the Finder's View menu for Show/Hide Toolbar or Show/Hide Sidebar. Are you inadvertently pressing those keyboard shortcuts?

I have never lost Finder Window Sidebar or Toolbar panels unless I deliberately choose that effect.

There are no Apple employees in these user-supported, public communities to answer why questions about Apple actions.

I wasn't judging you or being rude. I simply cannot reproduce what you state is happening across several different major versions of macOS, and if it were a bug, I would have encountered it and agreed with you.

The File Choosers that Chrome, VS Code, and all Apple applications use are not the real Finder, but a highly stylized Finder-like Framework (library) that emulates the Finder look. It may be an Apple Cocoa Framework, or it may be based on another visual Framework such as Qt that maps to a Cocoa Framework appearance.

In the context of an application Open/Save File panel, I haven't seen a missing sidebar or toolbar, but have seen some items missing from the sidebar — just not from Apple applications. I do not use Chrome or VS Code and any issues you have with those application's presentation of a File Chooser would be on the respective application developer, the UI Framework that they used, and how they coded that interface element. So, the bug (or coding omission of the application developer) is theirs, and not necessarily an Apple Finder bug.