"Create Similar" is one of my favorite commands in Revit. It is also one command that I use almost every day. I could create a lot of things quickly by selecting an existing elements around the space and keep building it.
Create similar is accessible both in project environment as well as family environment. In a project, you could literally select anything 2D or 3D, then go to your option under "Modify" tab and click the icon "Create Similar"
Or you can access it via "right click" and choose from the pop-up.
How about in Family? You can pretty much do the same; however, it only applies to traditional family.
Select the geometry, go to the option, click "create similar".
Or just "right click" after you have selected the geometry.
**The only thing I found in family that does not allow you to use "create similar" are model line and symbolic line. For some reasons, they just don't work with create similar.**
When it comes to adaptive component (or conceptual mass), once again, Revit just disable this nice feature from this type of family. If you "right click" by selecting the geometry, you actually get the same option from the pop-up window but it is grayed out. I suppose there is no time saving using "create similar" in adaptive component!?