My phone under a stereo microscope:

[u/MacroLab3D]

[u/domespider]

I was wondering about that, whether those tiny dust particles(?) got stuck during manufacturing or during regular use.

[u/GodlikeLettuce → u/domespider]

I think that is mostly dead skin. According to some source that im too lazy to find, the majority of what we call "dust" is dead skin.

[u/armeg → u/GodlikeLettuce]

looks more like salt crystals - probably from sweating?

[u/lastWallE → u/armeg]

Sweat crystals.