A medium format camera can also be a DSLR. An a6700 or an A7 is not a rangefinder, it's a mirrorless camera. You're mixing up characteristics.

DSLR means: it's digital, it uses one lens, and the visor image comes from the same lens that goes to the film, but reflected on a mirror to your eye (probably through a pentaprism, but that can vary), so you can see the same image that will be captured in the sensor.

A rangefinder is a camera where you look... through a rangefinder. It's a visor independent of the film/sensor, like a classic Leica. That means that the image you see is not exactly the same that will reach the sensor/film, as it will go through a different optical system. There are digital rangefinders like the M series Leicas.

Mirrorless means, it doesn't have a mirror to send the image to the visor. The typical mirrorless cameras nowadays get the visor image directly from the sensor, you don't look through a rangefinder. A rangefinder is mirrorless, but not every mirrorless is a rangefinder. Most digital mirrorless cameras aren't rangefinders.

A rangefinder (or mirrorless) camera is not inherently better than a DSLR. Mirrorless cameras can have simpler optical designs that achieve more quality easier, but there are better and worse cameras in both sides.