Showing a typical analogue SLR:
EXCEPT, that this illustration does not include the digital sensor or Pentaprism mirror sections; it demonstrates the older film SLR models.

The cross section of a DSLR camera differs primarily in internal component construction; where the film is replaced by a Pentaprism mirror assembly and its necessary hardware.
The DSLR camera uses an internal mirror assembly to reflect the image onto the view-finder. The viewfinder is the 2.5 display screen on the back of your DSLR Camera body.