What is the determining factor or factors for when the price and buy button appear first before the item description and for when it appears at the bottom? I can’t figure it out.

For example, at least on my PC this: https://dreadmaps.itch.io/cyberpunk-docks The price and buy button appear first

And this one: https://dreadmaps.itch.io/cindershard-ravine The price and buy button appear at the bottom

Please help.

It depends on how long your description is.

There should be two buttons on each page. The upper button can dissappear, if the description is displayed wide and short enough. If the upper button is missing, try resizing your window so it gets narrow to the point where the screenshots appear sideways.