Each of the makers is a little different in their policies, and long-term this is likely to have some impact on availability of third-party lenses for various mounts. Obviously, though, quantity of cameras sold also has an impact, as a third party maker needs a solid user base before they will commit.
- Canon EOS M is a closed system. Canon has not provided mount information to third-party makers. Given past history, it’s unlikely Canon will share mount information in the future, either. The Canon M mount has been reverse engineered by third parties, not licensed.
- Canon EOS RF and RF-S is a closed system. Canon has not provided mount information to third-party makers. Given past history, it’s unlikely Canon will share mount information in the future, either. The Canon RF mount has been reverse engineered by third parties, not licensed.
- Fujifilm’s XF mount is a semi-closed system. Fujifilm early on worked with a few companies, such as Zeiss, to get additional lenses available for their mount. At least two other third party lens makers seem to have some sanction from Fujifilm. However, other third party makers are still reverse engineering the mount.
- Micro 4/3 is a semi-open system. Companies can join the alliance and get the mount information. It's unclear what they have to agree to in order to get this information and provide officially sanctioned products, but we have many that have done so. Micro 4/3 is also unique in that it has two camera makers committed to it, which increases the overall market for such lenses. It's not surprising that we've seen the most activity in this mount so far. However, note that there are some differences between how OMDS and Panasonic bodies operate, so there can be small idiosyncrasies when it comes to things like lens image stabilization.
- Nikon CX was a closed system. Nikon did not provide mount information to third-party makers. Almost no third party lens makers attempted to reverse engineer it.
- Nikon Z is a closed system. Nikon has not provided mount information to third-party makers. Given past history, it's unlikely Nikon will share mount information in the future, either. Third party lens makers are reverse engineering their lenses to work on the Z mount (e.g. Viltrox, Yongnuo, both of whom have reverse engineered Nikon's autofocus communications).
- Samsung NX was a closed system. Samsung has not provided mount information to third-party makers that we know of, though there is a chance that they cooperated with local Korean companies (e.g. Samyang).
- Sony E and FE mounts are a semi-open system. Sony provides mount information to third-party makers who are interested in it and sign a license agreement. We have quite a few third party companies now making fully Sony-compatible lenses, including Samyang, Sigma, Tamron, Tokina, and Zeiss.
Note that some third party lenses that have appeared are just mount adaptions of existing lenses, expecially the manual focus ones. To get optimal size and performance, you really want a lens that was designed specifically to the cameras that it is intended for.