Nikon, Canon, and Sony all manufacture adapters to mount their SLR lenses (F-mount, EF mount, and A-mount, respectively) on their mirrorless systems (Z Mount, RF Mount, and E Mount, respectively). . Note that in some cases, all lenses, or all features of the lens (usually autofocus), are not retained or supported.
But as a general rule (with many small warnings for specific cases), you can use system lenses with large flange distances (for example, SLRs) on cameras with shorter flange distances (ie, without mirror). But not the opposite.
The diameter of the throat is another problem. If the diameter of the frame of a lens for long-flange system is much larger than that of the camera, the adapters may not have enough space to accept the wide lens. go up and always be small enough to go up to the camera. This is very specific to the pair of systems you are trying to interconnect.
These concerns are the same as when fitting a lens brand to another brand 's camera case. See Can I use the X Brand Lens on a Y Brand Interchangeable Lens Camera? for a more complete discussion of these issues.
Finally, if it is true that optically speaking, all you need is just an adapter (assuming that the distances between the flanges and grooves are conducive to adaptation), the electrical contacts and the communication with the lens must also be adapted. When adapting between brands, as this information is not available, companies such as Sigma, Tamron, Metabones, etc., must reverse engineer the electrical and communication protocols. Sometimes they do not get it totally, so there may be some bugs or incompatibility issues.