I highly recommend these You Tube video and do so without qualification or reservation.
The "BAE Systems" video the construction of an Astute class British RN nuclear attack submarine.
"Combat Approved" Part one and two courtesy the controlled [?] Russian media Russian Times.
More than anything else I guess emphasis on the infrastructure first to build a modern nuclear attack submarine. Enormous. A dedicated facility just required for fabrication.
Even beyond that consider the infrastructure and process required for retrofitting and refit, periodically, as part of the vessels normal life expectancy.
"Retrofitting refers to the addition of new technology or features to older systems."
"Refitting or refit of boats includes repairing, fixing, restoring, renewing, mending, and renovating an old vessel"
These nuclear attack submarines currently the most complicated thing made by man. The process of decommissioning and dismantling such a vessel also a dangerous, laborious, expensive and time-consuming process. Especially with regard to proper disposal of the atomic reactor.
NOBODY EVER SAID ANY OF THIS WAS GOING TO BE EASY!