DSLRs have a direct, optical path through the lens. (Well, a reflected direct path.) You are seeing the scene with your eyes.

Mirrorless cameras with a viewfinder actually use a small LCD screen — we call this an electronic viewfinder, or "EVF". You are seeing the image read from the sensor processed for viewing, not the scene itself.

Both approaches have advantages and disadvantages. In the past, when EVFs were small, slow, and low resolution, the winner was clearly the optical viewfinder of the DSLR. Now, as technology has improved, that's not necessarily the case. The question Disadvantages of electronic viewfinders? covers the issue in more depth — although note that it's from half a decade ago so EVFs have improved since then.

In some ways, the EVF is identical to what you might see on the rear screen of the same mirrorless camera, or to the rear screen of a point & shoot, or to that screen on a DSLR in live view. All of these work the same way. However, the EVF lets you hold the camera to your eye, which has advantages for stability, works in all lighting conditions, and can be easier for framing. (Plus, my eyes are about 45 years old and getting inflexible, yet I don't yet have bifocals, the viewfinder can be set to a comfortable virtual distance while the rear screen can't.) More on EVF vs rear screen at What are the benefits of EVFs over the rear LCD?.