I did engineering for some buildings for Bechtel and SAIC in Oak Ridge 15+ years ago. They had SCIFs. It wasn’t that big of a deal, just full height insulated walls, might have had Faraday cage elements, separate HVAC system, that sort of thing.
Key point operationally was no telecom wiring going in, and I presumed they had procedures to control data storage in/out.
Oh, and I think they had “airlocks” with badge in/badge out. Can’t remember if that was these SCIFs, or some other facilities we did.
One of the most juvenile and fun things to do was to call out “UNCLEARED ONBOARD!” and cover up any screen and paper to prevent the viewing of said info by the uninitiated (aka uncleared.)
Who remembers?