In this blog about bed linens, there is such a sentence:
A top sheet is literally a flat sheet which bed users lie beneath.
Isn't a sheet something a person lies on? Why does the writer use 'beneath' rather than 'on'?
Because the top sheet is on top. The sleeper lies on the bottom sheet, and the top sheet lies on them.
When you say the sleeper lies beneath the top sheet, that means the top sheet lies on top of them.
"A bottom sheet is laid above the mattress, and may be either a flat sheet or a fitted sheet (a sheet which has been sewn with elastic or a drawstring along the hem of the top, sides and bottom of the sheet to prevent it from slipping off of the mattress). A top sheet, in the many countries where they are used, is a flat sheet, which is placed above a bottom sheet and below other bedding."
The sleeper goes in between them.