What's the advantage of a DSLR's "previewer" viewfinder, since I have observed that mirrorless cameras appear to have the same thing?

Question

I have read some articles about DSLR and mirrorless cameras, and one thing all people insist on is that:

DSLRs use the same design as the 35mm film cameras from the past. A mirror inside the camera body reflects the light coming in through the lens up to a prism, and into the viewfinder for you to preview your shot.

source: beachcamera.com

But I have also seen that we can preview photos on the Canon M50, for example, which is a mirrorless camera. Here a photo of the Canon M50 with the previewer at the top of the device: https://www.imaging-resource.com/PRODS/canon-eos-m50/Z-M50-BACK.JPG

So what it is the point, when it seems also mirrorless camera also allows one to preview photo using a kind of lens, like a classic camera?

My question is slightly different to the possible duplicate proposals because I assume that this is the same thing, the other question already know it is a different thing, so my question begins from different assumptions, hence difference emphasizing in the answers. That said the two questions are complementaries I think, once you know why it's different you can dive in the advantage of one or another.

Answer 1

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?.