The basic idea is that you enter a shortcode into the post/page content.
There is a hughe number of possibilitied to show a single image, a list of (sub)album covers, a gallery of thumbnails, a slideshow etc. etc. All by a relative simple shortcode.
To make things simple, the editor has a so-called shortcode generator, that generates and inserts the shortcode that corresponds with your selection(s) in a dialog.
If you use the new block editor, aka Gutenberg, click the + icon to add a new block, scroll down in the menu and click on the WPPA Shortcode tab.
This shows you two icons: an album cover and a camera icon, The album cover icon opens the dialog for the general – full featured – shortcode generator dialog, the camera icon opend the dialog for the single image selection dialog. Answer the question(s)/make the required selection(s) and click the Ready button. You will see the preview of what the shortcode will do.
The old style classic editor has the same generator icons on top of the edit window in Visual mode.
There are various ways to display a single image, without caption or with a caption like standard wp images or with an extended caption. If you use the single photo shortcode genrator (via the camera icon), you can configure the way it will render in Table IX-L of the Photo Albums -> Settings admin page.
Alternatively you can modify or enter the shortcode ‘manually’ in html mode in both editors.
Full documentation of wppa shortcodes is here: https://wppa.nl/shortcode-reference/