Transient views described awkwardly here so I will summarize:
Cmd+P, to see a list of files in your project if you arrow over the file name in the list that is already open in a tab it should put that file into focus, pressing enter will make that file permanently active, but if you exit without selecting that file it wont become permanently active.
Also semi-transient view described quite well here:
Files opened from the sidebar create semi-transient views. Unlike transient views, semi-transient views show up as a new tab. You will be able to tell semi-transient views from other views because their tab text is shown in italics. When a new semi-transient view is opened, any existing semi- transient view in the same pane gets automatically closed.