Alaska Moose, it wouldn't be surprising if I did misinterpret what you had written since I am pretty much totally unfamiliar with Canon and its offerings, but the impression I had was that you were saying that a Canon user could purchase an adapter and use that same adapter on both his or her mirrorless body and DSLR body. ?? Is that not the case?
From what I understand about Nikon, their FTZ adapter works only on the Z series mirrorless camera bodies with some, but NOT all of their vast selection of older lenses in the F-Mount series (one thing which gave me pause when making some important decisions a year or so ago). Yes, someone can slap the FTZ adapter and one of their not-so-old F-mount lenses on to a current Z Series mirrorless camera body and get satisfactory results. People here and people on Nikon Cafe have demonstrated that this works quite nicely. Unfortunately, with older Nikon lenses, one has to make compromises such as no longer having AF or worse, simply not having the lens compatible at all with the FTZ adapter. This was the situation I faced and it was unacceptable to me so I switched to a system which would work for me.
The Canon adapters are similar to the Nikon ones in that once mounted on the mirrorless R, R5, and R6 I can use older Canon EF lenses that are designed for DSLR canon cameras. The new cameras (R, R5, and R6) have "R" mounts, while older Canon lenses have EF mounts.
Sigma, Tamrom, and other lens manufactures produce lenses with EF mounts as well as mounts for all other camera brands. I have no idea how well the Nikon adapters work on the Z-series cameras, but the three Canon adapters work perfectly with any lens designed with an EF mount.
All my Canon lenses are EF primes (I don't have any zoom lenses), so I am using them "adapted" to the R6, and without adapter on a couple of Canon DSLR cameras that I also have. However, the RF 100-500mm lens that I just purchased can only be used with the R, R5, and R6, and not with my two Canon cameras.
Also, all EF mounts have electrical contacts, while a lot of old lenses don't. In this case one can only focus the lens manually. The link to the photography forums I posted above can be used as research relating to the used of "adapted" lenses with Canon, Nikon, Sony, and other cameras.