Blisk attachment to shaft inside turbine

I am trying to learn more about turbines for the heck of it on this snowy afternoon and came across the Blisk. Combination of blade and disk.

I am looking for how a blisk is attached to the shaft in a steam or other turbine. I am finding tons on construction of the blisk, varying fin designs and construction, material analysis, and varying pitch. But everything that I look at shows the inside of the blisk as a smooth surface hollow ring.

Is the blisk pressed onto the shaft then welded?
Or am I missing some type of spline connection?
Or is there some type of disk in the center not being shown that the blisk slides against with pins or bolts being used to attach the two?
Or is the setup some type of hub with many blisk being pressed together "locking" to form a hollow hub of sorts. Which seems like it would cause increased rotational forces vs a shaft design.

Thank you for any help.