Since mobile phones and tablets are made in all different shapes and sizes, it's not possible to simply drag and drop elements on a screen so that they are positioned on a screen like you would in a Google Slides document, which is set to a fixed height and width.
To position elements of an app in Thunkable, you'll have to manage the spacing between elements with a few powerful properties. You will also find some helpful examples in the design templates and sample apps.
You can space elements within a Screen, Row or Column using two powerful properties
Horizontal Alignment and
Horizontal Alignment in a Screen, Row or Column
Vertical Alignment in a Screen, Row or Column
To create space around a visible component like an Image, you can use the
padding that are found in the Advanced section of the component properties. In most cases, you can add a margin OR padding to create space around an element -- they are interchangeable and can be set both in pixels and percent of the height or width. The fastest way to see what makes the most sense is to live test on your phone.