... after opening a folder with multiple files- if I then open a file and decide that is not the one I want - and then close the file- to move onto the next file- I have to re-open the entire folder...

There are a variety of better ways.

• Open the folder whose contents you wish to view, and click the "back" button at the top of that window to navigate back to the previous one.
• Or, open the folder and navigate back to the previous one with the Path icon in the window toolbar
• Open the folder in a new tab: hold a Control key while selecting the folder and select File > Open in New Tab
• Open the folder in a new window: hold an Option key (or two-finger tap) and select Open in New Window from the dropdown contextual menu

There are probably other methods.

macOS User Guide - Apple Support

The green dot toggles between Full Screen Mode and Windowed Mode.

Unless you want the Finder to monopolize the entire display you will probably want to be in Windowed Mode.

If you prefer the Finder occupies the entire display (and assuming you are not using an external monitor for additional desktop space) then you will probably want to use multiple tabs for those additional folders.

You can also choose View > As Columns, which enables you to see the folder structure and contents in a left-to-right manner.

As you can see... there are many ways to accomplish what you seek.

❤️

2015-ish? That's practically new 😄