High resolution pictures get pixelated in photoshop

What you need to understand is that zoom % in Photoshop has nothing to do with size. It is the relationship between image pixels and screen pixels. At 100%, one image pixel is represented by exactly one physical screen pixel.

At 500%, each image pixel is represented by 25 screen pixels! That's pretty big, almost the size of a small icon. So yes, naturally it gets pixelated.

Photoshop only works with pixels, and ppi, pixels per inch, is not a property of the file. It's a standard formula for pixel density on paper, used to determine the physical size in print of a given file. As size goes up, pixel density goes down. You only have so many pixels.

Ppi in itself means nothing. A file can be 1 ppi or 1247 ppi and it's the same file. To have any meaning, ppi needs to be expressed at a certain print size, or to a certain pixel size. Pixels per inch is an equation with three variables. If you have two variables, you can calculate the third.

On screen, ppi doesn't apply. A display has its own native pixel grid. There's no such pixel grid on paper, so one has to be defined. That's ppi.

What are the dimensions (in pixels) of your high-resolution images?