flash: Are the Yongnuo flashes interchangeable between the DSLR or are they specific to the brand?

If they are flash compatible with TTL, they are specific to the brand. And they only come in Canon and Nikon flavors. Manual only models are not brand specific and will work in any ISO compatible shoe.

It is not a 100% reliable guide (for example, the YN-500EX is an exception), but in general, any model that ends in 0 is only manual and has no mark. Any model that does not end in 0 is likely to be compatible with TTL and can be only for Canon, or it can be Canon or Nikon.

If the Canon and Nikon versions of a Yongnuo TTL model exist (for example, YN-568EX, YN-565EX), the Canon version will have silver letters in it; The Nikon version will have golden letters.

There are many rare Amazon / eBay listings for Yongnuo flashes that are the Canon version that also say they are TTL compatible with Nikon cameras. My personal opinion is that these are lists of people who simply move many units of many cheap Chinese products, and actually do not know anything about the flashes. And in a strange way, the assertion of compatibility with iTTL is true, because the flashes designated with "EX" of Yongnuo can used as CLS slaves in Nikon's optical wireless shooting system as well as in Canon's wireless eTTL, but will not function as an iTTL flash on a camera or on a TTL radio shoe unless they have the Nikon pin placement in the foot.

Check the pins of the feet.

The pins will always tell you what kind of flash you are dealing with. If there is a single pin to communicate the sync signal in the center of the foot, it is a manual and brand flash. The specific models of the brand with TTL capacity will have pins on the foot that will match the location of the shoe contact for the specific brand:

Canon hotshoe vs Nikon hotshoe

So, for example, as a Nikon shooter, you want a YN-568 with a foot that looks like this:

YN-568EX for Nikon foot

In the YN-622 triggers, you will notice that the same applies to the pins on the feet of those triggers, as well as the step-up shoes.

With Yongnuo hand triggers (RF-60x triggers), however, this is only maintained on the feet (they use a TTL signal to wake up, but only to wake up); The step shoe at the top has a modified contact location that covers the activation signal in both patterns. This is the reason why they do not make the full TTL step, but the YN-622 do it.