Every time I pose the above question as though it were a URL entered directly into my browser, I always get advice or videos telling me to go to slide master. But my question is about a PARTICULAR desired type of placeholder that I want to add to a PARTICULAR slide in a PARTICULAR presentation! Thanks in advance for your help.

I'm still a powerpoint newbie. Should I be establishing a new slide master for each possible new situation that I encounter?

Placeholders can only be added to slide layouts. You need to add a new layout to the current master, add your place holder to that new layout, then create a new slide from that layout.

Thsnks. If I understand correctly, then, every slide, no matter how unique it may seem to its author, must be an instance of a recognized master type?

Yes, though the term “Master” has a specific meaning in PowerPoint. A PPT template, or presentation, should ideally have only one master but may have many layouts. Slides are created from the Slide Layouts, not the Slide Master. If it has been set up correctly the layouts inherit their styling from the Slide Master.