PowerPoint and indeed all Microsoft Office programs allow you to work with a particular object only if it is selected. For example, you select a shape to make changes to the shape. You similarly select a chart to edit the chart. And yes, you can select a text container object such as a text placeholder, a text box, or even a shape, and then make changes to its position, formatting, size, etc. However, this changes the entire object, and leaves the actual text content within that object largely unchanged. To make changes to the actual text, you first need to select the text separately and then make changes by using the options available within the Home tab of the Ribbon or the Mini Toolbar.
To learn more, choose your version of PowerPoint. If we do not have a tutorial for your version of PowerPoint, explore the version closest to the one you use.